FemWIKI - Recent changes [en]

Attack rate

Bosmana fem — Mon, 18 May 2026 13:09:15 GMT

Attack rates and case fatality

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	=Attack rates and case fatality=		=Attack rates and case fatality=
	Attack rates and case fatality rates are examples of how the epidemiological jargon may be confusing.		Attack rates and case fatality rates are examples of how the epidemiological jargon may be confusing.

	==Attack rates==		==Attack rates==
	# Attack rates are actually [[Risks and rates\|risks]] (or "incidence proportion" or "cumulative incidence")		# Attack rates are actually [[Risks and rates\|risks]] (or "incidence proportion" or "cumulative incidence")
	# Attack rates are often expressed as a percentage.		# Attack rates are often expressed as a percentage.
	An attack rate is not an [[incidence rate]]. It is actually a risk (also called incidence proportions), and the time contribution of each individual is not included in the denominator.		An attack rate is not an [[incidence rate]]. It is actually a risk (also called incidence proportions), and the time contribution of each individual is not included in the denominator.
			The denominator consists of the number of people present at the beginning of the outbreak, disregarding those who will leave, develop illness, or die. This means that the cases (numerators) are also included in the denominator: it is, therefore, a true proportion.
			In outbreaks of short duration, the attack rate is frequently used instead of risk or incidence proportion. We often refer to "food-specific attack rates in a foodborne outbreak." In this circumstance, the denominator will consist of the number of people who ate a specific food, while the numerator will consist of the number of people who ate that food and became ill.

	The ~~denominator consists~~ of the number of ~~people present at~~ the ~~beginning~~ of the ~~outbreak, disregarding those who will leave, develop illness, or die~~. ~~This means that~~ the ~~cases (numerators) are also included in~~ the denominator~~: it is, therefore, a true proportion~~.		===Secondary attack rate===
			The '''secondary attack rate''' measures the spread of disease from a known case to susceptible contacts within a defined group, such as a household, classroom, or ward. It is calculated as the number of new cases occurring among contacts of a primary case, divided by the total number of susceptible contacts, during the incubation period following exposure. The primary case itself is excluded from both the numerator and the denominator.

	~~In outbreaks of short duration,~~ the ~~attack rate is frequently used instead~~ of ~~risk or incidence proportion. We often refer to "food-specific attack rates~~ in a ~~foodborne outbreak." In this circumstance~~, the ~~denominator will consist~~ of ~~the number of people who ate a specific food~~, ~~while the numerator will consist of the number of people who ate that food and became ill~~.		Secondary attack rates are particularly useful for assessing the transmissibility of an infectious agent in a closed setting, and for evaluating the effectiveness of interventions such as vaccination, prophylaxis, or isolation measures.

	=Case fatality, rates, and ratios: all the same?=		=Case fatality, rates, and ratios: all the same?=

Category:Epidemic intelligence

Bosmana fem — Mon, 18 May 2026 13:07:40 GMT

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	Epidemic intelligence integrates indicator-based and event-based ~~components. ‘Indicator-based surveillance’ refers to structured data collected through routine [[~~surveillance ~~principles\|surveillance systems]]~~. ~~The ‘[[event-based surveillance]]’ refers~~ to ~~unstructured data gathered from formal and informal sources, such~~ as ~~the media~~ and ~~scientific publications. The purpose of both these components of epidemic intelligence is~~ to ~~quickly identify any event which might become a public concern~~. Epidemic ~~Intelligence~~ covers [[Formal Risk Assessment\|risk assessment]] and risk monitoring.		Epidemic intelligence is the systematic detection, verification, analysis, and communication of information about events that may pose a threat to public health. It integrates two complementary components: indicator-based surveillance and event-based surveillance. Both aim to detect public health threats as early as possible and to monitor known threats until they are resolved. Epidemic intelligence covers both [[Formal Risk Assessment\|risk assessment]] and risk monitoring.

	~~[[File:eimodelecdc.png\|600px\|frameless\|none]]~~		== Components of epidemic intelligence ==


	Epidemic intelligence aims to produce timely, validated, and actionable intelligence on events related to communicable diseases or of unknown origin that interest public ~~health and~~ health authorities. The process can be divided into early detection of new threats and ~~events and~~ monitoring ~~the~~ already identified ~~threats~~, including potential threats.		'''Indicator-based surveillance''' refers to structured data collected through routine [[surveillance principles\|surveillance systems]]. These systems gather predefined indicators on identified risks, emerging risks, and non-human health-related risks (such as environmental or zoonotic factors).

			'''[[Event-based surveillance]]''' refers to unstructured information gathered from formal and informal sources, including official reports, the media, scientific publications, and rumours circulating at national or international level.

			The two components are complementary. Indicator-based surveillance provides continuous, comparable data on known threats over time. Event-based surveillance enables rapid detection of unexpected or emerging threats that structured systems may miss, often before they are captured by routine reporting.

			[[File:eimodelecdc.png\|600px\|frameless\|none\|ECDC Epidemic Intelligence model]]

			== Signals and events ==

			A frequent source of confusion in epidemic intelligence is the distinction between a '''signal''' and an '''event'''.

			A '''signal''' is a piece of raw information emerging from screening of formal or informal sources — a news item, a rumour, a hospital alert, an entry in a scientific publication — that ''may'' indicate a public health concern but has not yet been assessed for relevance or accuracy. Signals are abundant, frequently irrelevant, and often unverified.

			An '''event''' is a signal (or cluster of signals) that has passed filtering and is judged to be potentially relevant to public health: for example, an unexplained cluster of illness, an unusual outbreak, or a reported hazardous exposure. An event is a hypothesis worth investigating, but it is not yet confirmed.

			A '''validated event''' is an event that has been cross-checked against official or otherwise reliable sources and confirmed to be both real and adequately characterised. Only validated events proceed to risk analysis.

			In short, the process is a funnel of decreasing volume and increasing confidence: raw inputs are '''screened''' into signals, signals are '''filtered''' into events, and events are '''validated''' before analysis.

			== The epidemic intelligence process ==

			Epidemic intelligence aims to produce timely, validated, and actionable intelligence on events related to communicable diseases — or to disease of unknown origin — that are of interest to public health authorities. The process can be divided into the early detection of new threats and the monitoring of threats already identified, including potential threats.

			=== Early detection ===

	Early detection comprises six elements:		Early detection comprises six elements:

	# Screening news, official reports or notes, and rumours ~~relevant~~ from a European perspective to ~~distinguish the~~ meaningful ~~information~~ signals ~~by applying specified~~ criteria.		# '''Screening''' news, official reports, notes, and rumours from a defined (e.g. European) perspective in order to identify meaningful signals, using predefined criteria.
	# Filtering the ~~events~~ to ~~identify~~ potential public health events of ~~European~~ interest.		# '''Filtering''' the signals to retain only those that represent potential public health events of interest.
	# Validating ~~the~~ events ~~that originate~~ from unofficial sources by cross-checking ~~with~~ official ~~and/~~or reliable ~~media~~ sources to ~~ensure~~ that the event ~~detected~~ is real and ~~fully~~ understood.		# '''Validating''' events originating from unofficial sources by cross-checking against official or otherwise reliable sources, to confirm that the event is real and adequately understood.
	# A validated event ~~will then be analyzed~~ to capture the full ~~information about the event~~, including epidemiological data, exposure ~~facts~~, and contextual ~~information~~.		# '''Analysing''' the validated event to capture the full picture, including epidemiological data, exposure information, and contextual factors.
	# ~~Based on the analysis, an assessment is made to estimate~~ the risk associated with the event.		# '''Assessing''' the risk associated with the event on the basis of the analysis (see [[Formal Risk Assessment\|risk assessment]]).
	# ~~Finally, communication~~ and ~~documentation of~~ the identified threats ~~are integral to epidemic intelligence~~ throughout the five steps ~~above~~.		# '''Communicating and documenting''' the identified threats. This element runs throughout the previous five steps and includes dissemination via ad-hoc communications, daily bulletins, and weekly bulletins.

	Monitoring identified threats is the active follow-up of all relevant information directly related to the threat. This iterative process continues until the threat is considered to have subsided or until all appropriate public health measures have been implemented.		=== Monitoring identified threats ===

	~~Of course, in~~ a rapidly evolving situation, professional ~~judgment should be exercised, and~~ it may be appropriate ~~under severe time constraints~~ to skip some of the above~~-mentioned steps~~ to share information quickly. However, if epidemic intelligence ~~can be~~ gathered systematically, as described ~~above~~, the outcome is a better-informed decision and more effective action.		Monitoring is the active follow-up of all information directly relevant to an identified threat. It is an iterative process that continues until the threat is considered to have subsided or until all appropriate public health measures have been implemented.

			== Practical considerations ==

			In a rapidly evolving situation, professional judgement is essential. Under severe time constraints it may be appropriate to skip or compress some of the steps above in order to share information quickly. However, when epidemic intelligence is gathered systematically as described, the outcome is a better-informed decision and more effective action.

	<div style="display: inline-block; width: 25%; vertical-align: top; border: 1px solid #000; background-color: #d7effc; padding: 10px; margin: 5px;">		<div style="display: inline-block; width: 25%; vertical-align: top; border: 1px solid #000; background-color: #d7effc; padding: 10px; margin: 5px;">
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	: Arnold Bosman		: Arnold Bosman
	</div>		</div>


	[[Category:Assessing the burden of disease and risk assessment]]		[[Category:Assessing the burden of disease and risk assessment]]

Transmissibility

Bosmana fem — Mon, 18 May 2026 12:53:26 GMT

Show changes

Zoonosis

Bosmana fem — Mon, 18 May 2026 12:40:45 GMT

Created page with "== Zoonosis == '''Zoonosis''' (plural: '''zoonoses''') refers to any infectious disease that naturally transmits from animals to humans. The term derives from the Greek words ''zoon'' (animal) and ''nosos'' (disease). Zoonotic diseases represent a major public health burden globally, with approximately 75% of emerging infectious diseases being zoonotic in origin.<ref>Jones, K. E., Patel, N. G., Levy, M. A., et al. (2008). Global trends in emerging infectious diseases. N..."

New page

== Zoonosis ==

'''Zoonosis''' (plural: '''zoonoses''') refers to any infectious disease that naturally transmits from animals to humans. The term derives from the Greek words ''zoon'' (animal) and ''nosos'' (disease). Zoonotic diseases represent a major public health burden globally, with approximately 75% of emerging infectious diseases being zoonotic in origin.<ref>Jones, K. E., Patel, N. G., Levy, M. A., et al. (2008). Global trends in emerging infectious diseases. Nature, 451(7181), 990-993.</ref>

== Definition and Terminology ==

A '''zoonotic disease''' is defined as any disease or infection that is naturally transmissible from animals to humans. The term encompasses:

* '''Direct zoonoses''': Diseases that transmit directly from infected animals to humans
* '''Cyclozoonoses''': Diseases requiring both animals and humans for completion of the parasite lifecycle
* '''Metazoonoses''': Diseases transmitted to humans via insect or animal vectors

The '''zoonotic agent''' (the pathogen causing the disease) may originate from:
* Vertebrate animals (mammals, birds, reptiles, amphibians, fish)
* Invertebrate animals (insects, arachnids, mollusks)
* Environmental sources associated with animal populations

Humans in this context are considered ''accidental hosts'' or ''dead-end hosts'' for many zoonotic pathogens, meaning the pathogen cannot complete its lifecycle in humans alone and typically does not transmit person-to-person.

== Historical Context ==

Zoonotic diseases have shaped human history and civilization:

* '''Ancient period''': Plague, leprosy, and tuberculosis recognized as animal-associated diseases
* '''18th-19th centuries''': Edward Jenner's 1796 vaccination using cowpox (a zoonotic virus) demonstrated the principle of cross-species pathogen use for medical benefit
* '''20th century''': Recognition of zoonotic origins of influenza (1918 pandemic), HIV/AIDS (from non-human primates), and many others
* '''21st century''': High-profile zoonotic pandemics including SARS (2003), H1N1 influenza (2009), Ebola (2014-2016), Zika (2015-2016), and COVID-19 (2019-present)

The study of zoonoses became formalized with the concept of ''One Health'', recognizing the interconnection between animal health, human health, and environmental health.

== Classification ==

=== By Transmission Route ===

'''Direct transmission'''
* Bites and scratches (rabies from bats, raccoons)
* Contact with bodily fluids (Ebola, Marburg)
* Inhalation of aerosolized particles (tuberculosis from cattle, Q fever)
* Ingestion of contaminated food or water (salmonellosis, brucellosis)
* Skin contact (anthrax, ringworm)

'''Vector-mediated transmission'''
* Arthropod vectors: mosquitoes (dengue, yellow fever, Zika), ticks (Lyme disease, tick-borne encephalitis), fleas (plague), flies (sleeping sickness)
* Animal intermediate hosts: snails (schistosomiasis), crustaceans (paragonimiasis)

'''Environmental transmission'''
* Spores from soil or animal excreta (coccidioidomycosis, histoplasmosis)
* Contaminated environmental surfaces

=== By Epidemiological Pattern ===

'''Sporadic zoonoses'''
* Occur irregularly and unpredictably
* Examples: rabies, anthrax, most vector-borne diseases in non-endemic areas

'''Endemic zoonoses'''
* Maintained in animal populations with consistent human cases
* Examples: brucellosis in livestock regions, toxoplasmosis worldwide

'''Epidemic zoonoses'''
* Sudden increase in cases, often following animal-human contact changes or environmental disruption
* Examples: SARS, COVID-19, Nipah virus outbreaks

== Examples of Important Zoonotic Diseases ==

{| class="wikitable sortable"
|-
! Disease !! Zoonotic Agent !! Primary Animal Source !! Transmission Route !! Geographic Distribution
|-
| [[Rabies]] || Lyssavirus || Bats, dogs, raccoons, skunks || Bite wounds, saliva contact || Worldwide
|-
| [[Influenza]] || Influenza A virus || Birds, pigs || Respiratory droplets || Worldwide, seasonal
|-
| [[Tuberculosis]] || ''Mycobacterium bovis'' || Cattle, wildlife || Respiratory, ingestion || Worldwide
|-
| [[Brucellosis]] || ''Brucella'' species || Cattle, sheep, goats, pigs || Contact with tissues, unpasteurized dairy || Mediterranean, Middle East, parts of Asia
|-
| [[Plague]] || ''Yersinia pestis'' || Rodents || Flea bites || Central Asia, parts of Africa
|-
| [[Salmonellosis]] || ''Salmonella'' species || Poultry, reptiles, amphibians || Ingestion of contaminated food || Worldwide
|-
| [[Lyme disease]] || ''Borrelia burgdorferi'' || Deer, small mammals || Tick bite || Northern Hemisphere temperate regions
|-
| [[Dengue fever]] || Dengue virus || Monkeys, mosquitoes || Mosquito bite (''Aedes'' species) || Tropical/subtropical regions
|-
| [[Ebola virus disease]] || Ebola virus || Fruit bats, primates || Contact with blood/body fluids || Sub-Saharan Africa
|-
| [[Toxoplasmosis]] || ''Toxoplasma gondii'' || Cats, other animals || Ingestion of oocysts || Worldwide
|-
| [[Avian influenza]] || Influenza A virus (H5N1, H7N9, etc.) || Birds || Contact with infected birds || Worldwide in wild birds
|-
| [[Nipah virus infection]] || Nipah virus || Fruit bats, pigs || Contact with secretions, respiratory || Southeast Asia
|-
| [[HIV/AIDS]] || HIV || Non-human primates (SIV) || Contact with blood/body fluids || Worldwide, originated in Central Africa
|-
| [[SARS]] || SARS-CoV || Civets, bats || Respiratory droplets || Primarily East/Southeast Asia
|-
| [[COVID-19]] || SARS-CoV-2 || Bats (proposed), possibly intermediate hosts || Respiratory droplets || Worldwide pandemic
|}

== Risk Factors for Zoonotic Disease Transmission ==

=== Animal-Related Factors ===

* '''Pathogen prevalence in animal populations''': Higher infection rates in reservoir species increase transmission risk
* '''Animal reservoir size''': Larger populations support sustained pathogen maintenance
* '''Animal behavior''': Nocturnal animals reduce incidental human contact; animals entering human habitats increase risk
* '''Wildlife-livestock interface''': Contact between wild and domestic animals facilitates spillover
* '''Animal trade and transport''': Movement of infected animals to new geographic areas

=== Human Behavioral Factors ===

* '''Occupational exposure''': Hunters, farmers, veterinarians, laboratory workers, wildlife handlers
* '''Food practices''': Hunting and consuming bushmeat, consuming raw/undercooked meat, consuming unpasteurized dairy
* '''Pet ownership''': Close contact with companion animals
* '''Ecological practices''': Deforestation, habitat destruction that brings humans and wildlife into contact
* '''Travel and trade''': Movement to regions with endemic zoonoses or importation of infected animals/products

=== Environmental and Social Factors ===

* '''Ecological disruption''': Land-use changes, deforestation increase wildlife contact
* '''Climate change''': Alters animal distributions and breeding cycles, expanding vector ranges
* '''Socioeconomic status''': Limited access to healthcare, sanitation, and preventive measures
* '''Urbanization''': Dense human populations in proximity to wildlife reservoirs
* '''Armed conflict''': Disrupts disease surveillance and containment systems

== Mechanisms of Spillover ==

Zoonotic disease emergence typically involves several steps:

# '''Exposure''': Humans contact infected animals or contaminated environments
# '''Infection''': Pathogen infects human host cells
# '''Establishment''': Pathogen replicates and establishes infection
# '''Transmission''': Infected human transmits to other humans (not required for all zoonoses)
# '''Adaptation''': Pathogen evolves to become more suited to human hosts (occurs over multiple generations)

The [[basic reproduction number]] (R₀) in animals versus humans determines whether a zoonotic disease can sustain human-to-human transmission chains and cause epidemics.

== Public Health Significance ==

=== Disease Burden ===

* Approximately 60% of known human infectious diseases are zoonotic
* Approximately 75% of emerging infectious diseases have animal origins
* Annual economic losses from zoonotic disease and zoonotic animal disease exceed $220 billion USD globally<ref>Burns, A., van Panhuis, W., & Sazawal, S. (2016). The hidden pandemic of animal diseases. Chatham House Review.</ref>

=== Pandemic Risk ===

Zoonotic pathogens account for most pandemic threats:
* 1918 H1N1 influenza pandemic: ~50-100 million deaths
* 2009 H1N1 influenza pandemic: ~100,000-400,000 deaths
* HIV/AIDS pandemic: >40 million deaths to date
* COVID-19 pandemic: >7 million deaths (official count, likely underestimated)

=== Healthcare Burden ===

* Increased hospitalizations and healthcare costs
* Potential for rapid spread in healthcare settings
* Challenges to diagnostic and treatment infrastructure

== Prevention and Control ==

=== Individual-Level Measures ===

* '''Hand hygiene''': Regular handwashing, especially after animal contact
* '''Food safety''': Thorough cooking of meat, pasteurization of dairy
* '''Protective equipment''': Appropriate use of gloves, masks, and clothing in occupational settings
* '''Vaccination''': Where available (rabies post-exposure prophylaxis, avian influenza vaccines)
* '''Vector control''': Insect repellents, bed nets, permethrin-treated clothing against arthropods
* '''Animal precautions''': Avoiding wild animals, safe handling of pets

=== Community-Level Measures ===

* '''Disease surveillance''': Early detection of zoonotic disease cases and animal disease outbreaks
* '''Contact tracing''': Identification of exposed individuals
* '''Isolation and quarantine''': Separating infected individuals
* '''Vector control programs''': Mosquito spraying, tick control in endemic areas
* '''Public education''': Communication about zoonotic risks and prevention

=== Population-Level Measures ===

* '''One Health approach''': Coordinated efforts across human health, animal health, and environmental sectors
* '''Wildlife management''': Population control of reservoir species where appropriate
* '''Habitat modification''': Environmental management to reduce human-wildlife contact
* '''Biosecurity''': Controls on animal movement and trade
* '''Regulatory frameworks''': Laws governing bushmeat trade, pet trade, animal agriculture practices
* '''Climate adaptation''': Strategies to address climate-driven changes in disease distribution

=== Healthcare System Preparedness ===

* '''Diagnostic capacity''': Ability to rapidly identify zoonotic pathogens
* '''Infection control''': Protocols for managing zoonotic disease patients
* '''Supply stockpiles''': Maintenance of personal protective equipment and medical countermeasures
* '''Healthcare worker training''': Recognition and management of zoonotic diseases
* '''Research and development''': Investment in vaccines, therapeutics, and diagnostics

== Emerging Zoonotic Diseases ==

=== Factors Driving Emergence ===

* '''Ecological changes''': Habitat loss, climate change, agricultural intensification
* '''Demographic change''': Growing human population, urbanization
* '''Behavioral change''': Consumption of novel animal foods, increased wildlife trade
* '''Technological change''': Increased travel and connectivity facilitates spread
* '''Pathogen evolution''': Natural mutation and selection of viral and bacterial populations

=== Notable Emerging Zoonoses ===

* '''Henipavirus infections''' (Nipah, Hendra): Spillover from fruit bats through intermediate animal hosts
* '''Filovirus infections''' (Ebola, Marburg): Fruit bat-to-human spillover with secondary transmission
* '''SARS and COVID-19''': Suspected bat coronavirus origin with potential intermediate hosts
* '''Antimicrobial-resistant bacteria''': Including methicillin-resistant ''Staphylococcus aureus'' (MRSA) from livestock
* '''Novel arboviruses''': Zika, chikungunya expanding into new regions due to vector range expansion

== The One Health Approach ==

The '''One Health''' framework recognizes that human health, animal health, plant health, and environmental health are interconnected. This integrated approach to zoonotic disease:

* Involves collaboration between human medicine, veterinary medicine, and environmental sciences
* Addresses shared infectious disease threats at the human-animal-environment interface
* Promotes cross-sector communication and coordinated surveillance
* Advocates for sustainable food production and wildlife management
* Seeks to balance economic development with disease prevention

Organizations implementing One Health include [[World Health Organization|WHO]], [[Food and Agriculture Organization|FAO]], [[World Organisation for Animal Health|OIE]], [[United Nations Environment Programme|UNEP]], and national governments.

== Controversies and Debates ==

=== Origins of Pandemic Pathogens ===

The animal origin of certain pathogens, particularly SARS-CoV-2, remains contested. Key debates include:

* Evidence for and against animal spillover origins
* The role of intermediate hosts in spillover events
* The potential for laboratory accidents in pathogen transmission
* Geographic location of initial spillover events

=== Balancing Wildlife Conservation and Disease Control ===

* Tension between protection of endangered species and disease control measures
* Questions about when and how to manage wildlife populations
* Ethics of culling animals to prevent disease spillover

=== Animal Agriculture and Zoonotic Disease ===

* Factory farming practices and antimicrobial resistance
* Regulation of livestock production systems
* Trade-offs between food security and disease prevention

== See Also ==

* [[Pandemic|Pandemics]]
* [[Emerging infectious disease]]
* [[Vector-borne disease]]
* [[Reservoir (epidemiology)]]
* [[One Health]]
* [[Spillover (disease)]]
* [[Wildlife disease]]
* [[Antimicrobial resistance]]
* [[Food safety]]
* [[Public health]]

== References ==

<references />

== Further Reading ==

* Jones, B. A., Grace, D., Kock, R., et al. (2013). Zoonosis emergence linked to agricultural intensification and environmental change. ''Proceedings of the National Academy of Sciences'', 110(21), 8399-8404.
* Morse, S. S., Mazet, J. A., Woolhouse, M., et al. (2012). Prediction and prevention of the next pandemic zoonosis. ''The Lancet'', 380(9857), 1956-1965.
* Woolhouse, M. E., & Gowtage-Sequeria, S. (2005). Host range and emerging and reemerging pathogens. ''Emerging Infectious Diseases'', 11(12), 1842-1847.
* World Health Organization. (2020). Antimicrobial resistance: Global Report on Surveillance. WHO Publications.

== External Links ==

* [https://www.who.int/news-room/fact-sheets/detail/zoonoses WHO Fact Sheet on Zoonoses]
* [https://www.cdc.gov/onehealth/index.html CDC One Health Page]
* [https://www.oie.int/ World Organisation for Animal Health]

{{Categories|Epidemiology|Infectious diseases|Public health|Zoonosis|One Health}}

__NOTOC__
__FORCETOC__

Stage 2: Systematically collecting information

Bosmana fem — Mon, 18 May 2026 12:39:21 GMT

Checklist 2: Basic disease information/determinants

← Older revision		Revision as of 12:39, 18 May 2026
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	** If not, what are routes of introduction, e.g. food/bird/animal/human?		** If not, what are routes of introduction, e.g. food/bird/animal/human?
	** Seasonal/temporal trends		** Seasonal/temporal trends
	* Reservoir (if zoonotic, which species affected – will animals be symptomatic?)		* [[Reservoir for infectious agents\|Reservoir]] (if [[Zoonosis\|zoonotic]], which species affected – will animals be symptomatic?)
	* Susceptibility: are specific risk groups at increased risk of exposure/infection, e.g.:		* Susceptibility: are specific risk groups at increased risk of exposure/infection, e.g.:
	** specific age groups (e.g. children, elderly);		** specific age groups (e.g. children, elderly);
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	** Novel transmission routes		** Novel transmission routes

	Basic disease information from standard textbooks should be supplemented by searching published and grey literature (including outbreak reports and surveillance data, guidelines, disease fact sheets, etc.). “A literature search should be a well-thought-out and organized search for all relevant literature published on a topic and is the most effective and efficient way to locate sound evidence on a subject.” (see http://www.nursingtimes.net/nursing-practice/217252.article). When time and resources are limited, a preliminary literature search should be undertaken to identify the key literature in the subject area. However, there will inevitably be a trade-off between time and sensitivity. Particular attention should be given to filtering the results,i.e. choice of subjects, timeframe, and restricting to ‘review’ articles – most citation databases offer the facility to filter searches in this way. A trained information specialist or librarian can help to identify the best way to use these options in databases and retrieve the appropriate records according to the questions. There are also sites with tutorials and guides providing help with the literature search, such as the London School of Hygiene & Tropical Medicine Library (see http://www.lshtm.ac.uk/library/help/help.html for further information). It should be acknowledged that a comprehensive systematic review will not be possible in the early stages of a rapid risk assessment; however, the need for such a review should be considered later when time and resources permit.		Basic disease information from standard textbooks should be supplemented by searching published and grey literature (including outbreak reports and surveillance data, guidelines, disease fact sheets, etc.). “A literature search should be a well-thought-out and organized search for all relevant literature published on a topic and is the most effective and efficient way to locate sound evidence on a subject.” (see http://www.nursingtimes.net/nursing-practice/217252.article). When time and resources are limited, a preliminary literature search should be undertaken to identify the key literature in the subject area. However, there will inevitably be a trade-off between time and sensitivity. Particular attention should be given to filtering the results,i.e. choice of subjects, timeframe, and restricting to ‘review’ articles – most citation databases offer the facility to filter searches in this way. A trained information specialist or librarian can help to identify the best way to use these options in databases and retrieve the appropriate records according to the questions. There are also sites with tutorials and guides providing help with the literature search, such as the London School of Hygiene & Tropical Medicine Library (see http://www.lshtm.ac.uk/library/help/help.html for further information). It should be acknowledged that a comprehensive systematic review will not be possible in the early stages of a rapid risk assessment; however, the need for such a review should be considered later when time and resources permit.

	=Published literature=		=Published literature=

Transmissibility

Bosmana fem — Mon, 18 May 2026 12:25:19 GMT

Infectiousness / Transmissibility

Show changes

Transmissibility

Bosmana fem — Mon, 18 May 2026 12:19:24 GMT

Created page with "= Infectiousness / Transmissibility = ==Definition== Infectiousness (or transmissibility) refers to the ability of a pathogen or infectious agent to spread from an infected person to others. It quantifies how readily a disease is transmitted within a population and is a fundamental parameter in epidemiological modeling and disease control strategy. Infectiousness is distinct from pathogenicity (the capacity to cause disease) and virulence (the severity of disease caused)..."

New page

= Infectiousness / Transmissibility =
==Definition==
Infectiousness (or transmissibility) refers to the ability of a pathogen or infectious agent to spread from an infected person to others. It quantifies how readily a disease is transmitted within a population and is a fundamental parameter in epidemiological modeling and disease control strategy.
Infectiousness is distinct from pathogenicity (the capacity to cause disease) and virulence (the severity of disease caused). A highly infectious disease may be mild, while a severe disease may be poorly transmitted.
==Key Parameters==
===Basic Reproduction Number (R₀)===
The basic reproduction number (R₀, pronounced "R naught") is one of the most important epidemiological metrics. It represents the average number of secondary infections that result from a single infected individual in a completely susceptible population, assuming no interventions or behavioral changes.
====Mathematical Definition====
R₀ is calculated as:
R₀ = β × c × d
Where:
· β = transmissibility (probability of transmission per contact)
· c = contact rate (average number of contacts per unit time)
· d = duration of infectiousness (average time a person remains contagious)
Interpretation
· R₀ < 1: The infection will die out naturally; each infected person infects, on average, fewer than one other person
· R₀ = 1: Endemic equilibrium; the infection sustains itself at a stable level
· R₀ > 1: The infection will spread; each infected person infects more than one other person on average
· Larger R₀ values indicate more readily spreading infections and greater transmission potential
====Examples of R₀ Values====
Disease R₀ Range Transmission
Seasonal influenza 0.9–2.0 Moderate
COVID-19 (original strain) 2.0–3.0 Moderate to high
COVID-19 (Delta variant) 5–8 High
COVID-19 (Omicron variant) 8–12 Very high
Chickenpox 10–12 Very high
Measles 12–18 Extremely high
Smallpox 5–7 High
Polio 5–7 High
HIV/AIDS 0.5–3 Low to moderate

===Effective Reproduction Number (Rₑ or Rₜ)===
The effective reproduction number (Rₑ or Rₜ, where t indicates time) represents the average number of secondary infections caused by a single infected individual in the current population at a specific time, accounting for immunity, interventions, behavioral changes, and non-susceptible individuals.
====Mathematical Definition====
Rₑ(t) = R₀ × s(t)
Where:
· R₀ = basic reproduction number
· s(t) = proportion of the population that is susceptible at time t
====Key Differences from R₀====
Aspect R₀ Rₑ
Population Completely susceptible Current population state
Immunity Not accounted for Accounts for existing immunity
Interventions Assumes none Includes interventions, vaccines, behavior changes
Time dependence Constant, intrinsic to pathogen Changes over time and with circumstances
Practical use Theoretical benchmark Operational monitoring during outbreaks

===Epidemiological Significance===
· Rₑ < 1: The outbreak is declining or under control
· Rₑ = 1: Steady-state transmission (cases neither increasing nor decreasing)
· Rₑ > 1: The outbreak is accelerating or expanding
Because Rₑ incorporates real-world conditions, it is the more practically useful metric for monitoring epidemics and evaluating intervention effectiveness.
==Factors Affecting Transmissibility==
====Pathogen Characteristics====
· Virus/bacteria shedding rate: How much pathogen is released by infected individuals
· Genetic mutations: Changes can increase transmissibility (e.g., variant emergence)
· Stability in environment: How long the pathogen survives outside hosts
· Infectious period: The window during which a person can transmit
· Asymptomatic transmission: Capacity to spread before or without causing symptoms
====Population and Environmental Factors====
· Contact patterns: Density of population, social structures, frequency of interactions
· Hygiene and sanitation: Hand washing, water quality, sanitation infrastructure
· Climate and seasonality: Temperature, humidity, seasonal behavior changes
· Vaccination coverage: Proportion of population immune through vaccination
· Prior infection: Natural immunity from previous exposure
· Age structure: Different age groups may have different contact patterns and susceptibility
· Healthcare access: Early detection and isolation reduce transmission
====Behavioral Factors====
· Mobility and travel: Movement patterns spread pathogens across regions
· Social distancing: Reduces contact rates significantly
· Mask usage: Reduces transmissibility through respiratory droplets
· Isolation of sick individuals: Removes infectious people from contact with others
· Risk perception: Individual behavior changes based on perceived threat
==Measurement and Estimation==
===Direct Estimation===
In an early epidemic with complete contact tracing data:
R₀ ≈ (average number of secondary cases per infected individual)
====Statistical Methods====
· Generation time approach: Uses the serial interval and growth rate
· Contact tracing data: Tracks who infected whom
· Maximum likelihood estimation: Fits epidemic models to observed data
· Bayesian methods: Incorporates uncertainty and prior knowledge
===Real-Time Estimation===
Rₑ is estimated during epidemics using:
· Case incidence data: Number of confirmed cases over time
· Serial interval: Average time between case generations
· Smoothing techniques: Account for reporting delays and data variability
· Multiple methods: Cross-checking with different approaches improves reliability
====Common Estimation Methods====
· Wallinga-Teunis method
· Cori method
· Time-dependent methods using exponential growth rates
==Herd Immunity Threshold==
The herd immunity threshold is the proportion of the population that must be immune (through vaccination or prior infection) to prevent sustained transmission.
Herd Immunity Threshold = 1 - (1/R₀)
==Examples==
Disease R₀ Herd Immunity Threshold
Seasonal flu 1.3 23%
COVID-19 (original) 2.5 60%
COVID-19 (Delta) 6.5 85%
Measles 15 95%
Polio 6 83%

When vaccination coverage exceeds this threshold, the disease cannot sustain itself in the population, protecting even those not vaccinated (indirect protection).
Practical Applications
Outbreak Response
· Early assessment: Initial R₀ estimates inform intervention intensity
· Real-time monitoring: Tracking Rₑ determines if control measures are working
· Resource allocation: Higher R values indicate need for more aggressive response
Vaccination Strategy
· Coverage targets: Herd immunity threshold guides vaccination campaigns
· Booster decisions: Changing Rₑ indicates when immunity-boosting measures are needed
· Variant concerns: Increased R values prompt vaccine updates
Public Health Planning
· Hospital capacity: Higher R values predict more cases and healthcare burden
· Containment feasibility: R₀ < 1 suggests containment is possible; large R₀ suggests mitigation focus
· Intervention selection: Different interventions target different components of the R formula
Disease Modeling
· Epidemic projections: R values predict outbreak trajectory
· Intervention scenarios: Modeling shows how different measures affect R
· Long-term planning: Estimates inform pandemic preparedness
Limitations and Considerations
Assumptions and Challenges
· Homogeneous mixing: Actual populations have heterogeneous contact patterns (some people have many contacts, others few)
· Temporal variation: Contact patterns and susceptibility change over time
· Data quality: Relies on accurate case counts, testing, and reporting
· Reporting delays: Case reporting lags affect real-time estimates
· Uncertainty: Confidence intervals often wide during uncertainty
Heterogeneity
Real transmission is not uniformly random:
· Super-spreaders: Some individuals transmit to many more than average
· Superspreading events: Specific circumstances produce disproportionate transmission
· Spatial clustering: Transmission follows geographic and social networks
· Age and risk stratification: Transmission varies by age group and risk profile
Methodological Issues
· Estimation uncertainty: Different methods may yield different R values
· Generational overlap: Serial intervals difficult to estimate early in epidemics
· Incomplete data: Asymptomatic and undetected cases complicate estimates
· Non-stationarity: Changing conditions violate constant R assumptions
Historical Context and Evolution
The concept of R₀ emerged from mathematical epidemiology in the early 20th century, with major developments by:
· William Hamer (1906): Formulated the "mass action principle"
· Anderson and May (1980s): Developed comprehensive R₀ theory in population dynamics
· Modern applications: Real-time R estimation became standard during COVID-19 pandemic
The pandemic demonstrated both the utility and challenges of R-based monitoring, spurring improvements in methodology and real-time estimation techniques.
See Also
· [[Epidemic Curves and Growth Rate]]
· [[Serial Interval and Generation Time]]
· [[Vaccination and Immunization]]
· [[Contact Tracing]]
· [[Mathematical Epidemiology]]
· [[Pathogenicity and Virulence]]
==References==
· Keeling, M. J., & Rohani, P. (2008). Modeling infectious diseases. Princeton University Press.
· Wallinga, J., & Teunis, P. (2004). Different epidemic curves for severe acute respiratory syndrome reveal similar impacts of control measures. American Journal of Epidemiology, 160(6), 509-516.
· Fraser, C. (2007). Estimating individual and household reproduction numbers in an emerging epidemic. PLoS Medicine, 4(7), e300.
· Thompson, R. N., Stockwin, J. E., van Gaalen, R. D., et al. (2019). Improved inference of time-varying reproduction numbers during outbreaks. Epidemics, 29, 100356.

Last Updated: May 2026

Status: Complete

Difficulty Level: Intermediate to Advanced

Formal Risk Assessment

Bosmana fem — Mon, 18 May 2026 12:16:09 GMT

Defining the impact

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