Key definitions in infectious diseases epidemiology

Infectious disease epidemiology shares the same general conceptual framework as ‘non-infectious disease’ epidemiology. It seeks to understand the causes and distribution of infectious diseases in populations with the aim of controlling them. However, there are specific epidemiological concepts/terms that are mainly related to infectious diseases:

Agent

A disease agent is a biological agent that causes a disease. This can be a virus, bacterium, fungus, parasite, or other type of microorganism, as well as certain toxins and other substances that can cause disease. Disease agents can be transmitted from person to person, or through contact with contaminated surfaces, food, water, or other sources. They can cause a wide range of diseases, ranging from mild to severe, and can have serious consequences for individuals and communities. Some examples of disease agents include the influenza virus, which causes the flu; the bacterium Escherichia coli, which can cause food poisoning; and the parasite Plasmodium, which causes malaria.

Chain of transmission

The chain of transmission refers to the steps or stages through which a disease agent is transmitted from one person or host to another. It involves a series of events that occur in a specific order, starting with the source of the disease agent and ending with the infection of a new host. The chain of transmission can be broken at any point, which can help to prevent the spread of the disease. There are several key elements in the chain of transmission:

The source: This is the source of the disease agent, which can be a person, animal, or environment.
The reservoir: This is the place where the disease agent can survive and multiply.
The mode of transmission: This is the way in which the disease agent is transmitted from the source to the host. This can be through direct contact (such as through touching, kissing, or sexual contact), indirect contact (such as through contaminated objects or surfaces), or through the air (such as through coughing or sneezing).
The host: This is the person or animal that becomes infected with the disease agent.
The environment: This includes the physical, social, and cultural factors that can influence the transmission of the disease agent.

Breaking the chain of transmission involves interrupting one or more of these elements, which can help to prevent the spread of the disease. This can be achieved through measures such as vaccination, hygiene practices, quarantine, and other public health interventions.

Contagiousness

Contagiousness refers to the ability of a disease to be transmitted from one person or animal to another. A disease that is highly contagious can be transmitted easily and quickly, often with just brief contact or through the air. A disease that is less contagious may require more prolonged or close contact in order to be transmitted.

The contagiousness of a disease can depend on several factors, including the type of disease agent (such as a virus or bacterium), the mode of transmission (such as through direct or indirect contact, or through the air), and the susceptibility of the host (such as a person or animal). Some diseases, such as the common cold, are highly contagious and can be transmitted easily through contact with respiratory secretions or contaminated surfaces. Other diseases, such as HIV, are less contagious and require more specific modes of transmission, such as through sexual contact or sharing needles.

Understanding a disease's contagiousness is important in preventing its spread and controlling outbreaks. Public health measures, such as vaccination and hygiene practices, can help reduce a disease's contagiousness and prevent its transmission.

Epidemic curve

An epidemic curve is a graphical representation of the number of cases of a specific disease that occur over time. It is used to understand the spread and impact of an outbreak, as well as to identify patterns and trends in the data.

The epidemic curve is usually plotted on a graph, with the x-axis representing time (often in days or weeks) and the y-axis representing the number of cases. The shape of the curve can provide important information about the nature of the outbreak, such as the rate at which the disease is spreading and the population groups that are most affected.

There are several types of epidemic curves that can be used to represent different types of outbreaks:

A linear epidemic curve shows a constant rate of disease transmission over time.
An exponential epidemic curve shows a rapid increase in cases over time, often indicating a highly contagious disease.
A log-linear epidemic curve shows a slowing of the rate of disease transmission over time, often indicating that public health interventions are having an effect.
A logistic epidemic curve shows a slowing of the rate of disease transmission, followed by a plateau and then a decline, often indicating that the outbreak has reached its peak and is starting to decline.

Understanding the shape of the epidemic curve can help public health officials to identify the most effective interventions for controlling the outbreak and preventing further transmission of the disease.

Generation time

The time that elapses between the onset of symptoms in the primary case and the onset of symptoms in the secondary case. Generation time is a term used to describe the time it takes for a disease to be transmitted from one person to another or from one generation of hosts to the next. In the context of an epidemic, generation time can be an important factor in understanding the spread and impact of the disease and in identifying strategies for controlling the outbreak.

The generation time of a disease can vary depending on the specific disease agent and the characteristics of the host population. Some diseases have a relatively short generation time, meaning that they can be transmitted quickly and can lead to rapid outbreaks. Other diseases have a longer generation time, meaning they may take longer to spread and may have a slower impact on the population.

Understanding the generation time of a disease can help public health officials to identify the most effective interventions for controlling the outbreak and preventing further transmission of the disease. This can include measures such as vaccination, quarantine, and hygiene practices, as well as strategies for reducing the number of contacts between individuals in the population.

Herd immunity

Herd immunity, also known as community immunity, refers to the protective effect that occurs when a high proportion of a population is immune to a specific disease. This can be achieved through natural immunity (for example, by recovering from the disease) or through vaccination.

When a large proportion of a population is immune to a disease, it is more difficult for the disease to spread, because there are fewer individuals who are susceptible to infection. This can provide protection for those who are not immune, including those who are too young to be vaccinated, those who are unable to be vaccinated due to underlying health conditions, and those who have not yet had the opportunity to be vaccinated.

Herd immunity can be an important factor in controlling the spread of infectious diseases and preventing outbreaks. The level of immunity needed to achieve herd immunity varies depending on the specific disease and the characteristics of the population. Some diseases, such as measles, require a relatively high level of immunity (around 95%) in order to achieve herd immunity, while others, such as pertussis (whooping cough), require a lower level of immunity (around 80%).

It is important to maintain high vaccination rates in order to protect the population from infectious diseases and to maintain herd immunity. This can help to prevent outbreaks and protect those who are most vulnerable to serious illness or complications from the disease.

Host

A host is an individual or animal that is infected with a disease agent, such as a virus or bacterium. The host serves as a source of the disease, and can transmit the disease to other individuals or animals through the process of transmission.

The term "host" can also refer to the individual or animal that provides a habitat or environment for a particular disease agent. For example, a mosquito may be the host for a parasite that causes malaria, while a human may be the host for a virus that causes the flu. A primary host is where a parasite reaches maturity or passes its sexual stage. A secondary host is where a parasite is in a larval or asexual stage.

Understanding the role of hosts in the transmission of disease is an important aspect of epidemiology, as it helps public health officials to identify the sources of outbreaks and to develop strategies for preventing the spread of the disease. This can include measures such as vaccination, quarantine, and hygiene practices, as well as strategies for reducing the number of contacts between individuals in the population.

Incubation period

The incubation period is the time between when an individual is exposed to a disease agent (such as a virus or bacterium) and when they develop symptoms of the disease. The incubation period can vary depending on the specific disease, as well as the individual's age, immune system, and other factors.

During the incubation period, the disease agent may be multiplying and spreading in the body, but the individual is not yet experiencing any symptoms. This can make it difficult to identify the source of the infection, as the infected individual may not realize that they are carrying the disease and may be unknowingly spreading it to others.

Understanding the incubation period of a disease is important in terms of preventing the spread of the disease and controlling outbreaks. For example, if the incubation period is relatively long, public health officials may need to implement quarantine measures for a longer period of time in order to prevent the transmission of the disease. If the incubation period is shorter, more immediate interventions may be necessary to control the spread of the disease.

Index case

First case of a disease to be identified at the start of an outbreak. The index case is the first patient that indicates the existence of an outbreak. It does not necessarily mean that it was the outbreak's first case. Earlier cases may be found and are labeled primary case, secondary case, tertiary case, etc.

Primary case

The primary case (or source case, or patient zero), is the first case of a disease in an outbreak or epidemic. This individual introduced the disease agent to the population and gave rise to the outbreak or epidemic.

Latent period

The latent period is the time between the initial infection and the onset of infectiousness. In other words, it is the period during which the pathogen replicates within the infected individual's body, but the individual is not yet contagious.

Outbreak

Term used in epidemiology to describe an occurrence of disease greater than would otherwise be expected at a particular time and place. It may affect a small and localized group or impact upon thousands of people across an entire continent. Two linked cases of a rare infectious disease may be sufficient to constitute an outbreak.

Epidemic

Generally, ‘epidemic’ refers to large outbreak. But the difference between ‘epidemic’ and ‘outbreak’ remains subjective. Some have proposed that an epidemic is an outbreak that affects a region in a country of a group of countries.

Pandemic

Outbreak of disease around the globe.

Pathogen

A pathogen is any agent that can cause disease in a host organism. Pathogens are typically infectious agents such as viruses, bacteria, fungi, or parasites, but they can also be non-infectious agents such as physical agents (e.g., radiation) or chemical agents (e.g., toxins).

Pathogens can cause a wide range of diseases in humans and other animals, ranging from mild infections to serious, life-threatening illnesses. Some common examples of pathogens include the bacteria that cause tuberculosis and salmonella, the viruses that cause influenza and HIV, and the fungi that cause athlete's foot and ringworm.

Pathogens are able to enter the body of a host in a variety of ways, including through contact with infected bodily fluids (such as blood or saliva), through inhalation of respiratory secretions, or through ingestion of contaminated food or water. Once inside the body, the pathogen may begin to replicate and cause damage to cells and tissues, leading to the development of symptoms of disease.

Reproductive rate

The reproductive rate of an infection refers to the number of new infections that are generated by each infected individual during the course of their illness. It is often expressed as the basic reproductive number, or R0 (pronounced "R-naught"), which is the average number of new infections produced by a single infected individual in a population that is fully susceptible to the infection.

The reproductive rate is an important concept in the field of epidemiology, as it helps to predict the spread and potential impact of an infectious disease within a population. A high reproductive rate can indicate that an infection is highly contagious and may spread rapidly through a population, while a low reproductive rate may suggest that the infection is less easily transmitted.

The reproductive rate of an infection can vary widely depending on a number of factors, including the mode of transmission (e.g., respiratory droplets, contact with bodily fluids), the severity of the disease, and the effectiveness of control measures (such as vaccination or isolation of infected individuals).

To calculate the reproductive rate of an infection, epidemiologists use a variety of statistical and mathematical models that take into account the number of new infections, the length of the infectious period, and the size of the population. These models can help to identify the key drivers of transmission and inform the development of strategies to control the spread of the infection.

Transmission route

A transmission route refers to the way in which a pathogen (such as a virus or bacterium) is transmitted from one host to another. Transmission routes can vary depending on the specific pathogen and the characteristics of the host population.

Some common transmission routes for infectious diseases include:

Respiratory droplets: Many respiratory infections, such as the common cold and influenza, are transmitted through respiratory droplets that are released into the air when an infected person talks, coughs, or sneezes. Other people can inhale these droplets and then become infected.
Contact with bodily fluids: Some infections, such as HIV and hepatitis B, are transmitted through contact with infected blood or other bodily fluids. This can occur through sexual contact, injection drug use, or accidental exposure to contaminated needles or other medical equipment.
Food and water: Some infections, such as norovirus and salmonella, are transmitted through contaminated food or water. These pathogens can be present in undercooked or raw food, or water contaminated with feces.
Insects: Some infections, such as malaria and West Nile virus, are transmitted through the bites of infected insects, such as mosquitoes or ticks.
Fomites: Fomites are inanimate objects that can become contaminated with pathogens and serve as a means of transmission. Examples of fomites include towels, bedding, and other household items that can harbor pathogens and transmit them to other people.

Understanding the transmission route of an infection is important in the control and prevention of the disease, as it can inform the development of strategies such as vaccination, isolation of infected individuals, and the implementation of infection control measures.

Direct transmission

Direct and immediate transfer of infectious agents to a susceptible host. This may be through direct contact such as touching, biting, kissing or sexual intercourse, or by the direct projection of droplet (droplet spread) spraying onto eyes, nose or mouth of other people. Droplet spread is usually limited to short distances, such as 1 meter or less.

Vertical transmission

A specific direct transmission is between mother and child during pregnancy or childbirth.

Indirect transmission

Transmission of infectious organisms from a source through objects (vehicles) or insects (vectors).

Vehicle-borne

Infectious agents can reach susceptible hosts through transport on inanimate objects (=fomites) such as toys, handkerchiefs, soiled clothes, bedding, medical instruments, food, water, blood products or any other substance that can be contaminated. Some vehicles allow multiplication of the infectious agent (e.g. salmonella in food), though this is not always the case.

Vector-borne

When insects transfer infectious agents to susceptible hosts, they act as 'vectors' of the infection.

Airborne transmission

Microbial aerosols are suspensions of particles (fluid or solid) in the air consisting partially or wholly of microorganisms. They may remain suspended in the air for prolonged periods of time (as opposed to droplets that are too large in diameter and fall to the ground relatively fast). This transmission route works particularly efficiently for viruses such as the measles virus.

Reservoir

a reservoir is a place or host where a pathogen (such as a virus or bacterium) can survive, grow, and reproduce. A reservoir can be either a living organism (such as a human, animal, or plant) or an inanimate object (such as soil or water).

The concept of a reservoir is important in epidemiology because it helps to understand how infectious diseases are transmitted and how they can be controlled. For example, if the reservoir for a particular pathogen is an animal, such as a rodent or a bird, understanding the habits and habitats of that animal can help to identify potential sources of infection and implement control measures to reduce the risk of transmission to humans.

Some common examples of reservoirs for infectious diseases include:

Humans: Many infectious diseases, such as influenza and measles, have humans as their primary reservoir.
Animals: Many infections, such as rabies and West Nile virus, are transmitted to humans from animals. The animal reservoir for these infections may be wild animals, domestic pets, or livestock.
Water: Some infections, such as cholera and typhoid fever, are transmitted through contaminated water. In these cases, the water can act as a reservoir for the pathogen.
Soil: Some infections, such as tetanus and anthrax, are transmitted through contact with contaminated soil. In these cases, the soil can act as a reservoir for the pathogen.

Understanding the reservoir for an infectious disease is important for developing control measures and designing effective interventions to prevent the spread of the infection.

Susceptibility

A susceptible individual (sometimes known simply as a susceptible) is a population member at risk of becoming infected by a disease.

Source

The source is the place or host from which the pathogen is transmitted to another host.

To understand the difference between a reservoir and a source, it can be helpful to think of a reservoir as a place where the pathogen is "stored" and a source as a place where the pathogen is "released." For example, a person with an infection may act as a reservoir for the pathogen (storing it within their body), while the respiratory secretions they release when they sneeze or cough may act as the source of the infection (releasing the pathogen into the environment).

In some cases, the reservoir and the source for an infectious disease may be the same. For example, if a person with an infection sneezes or coughs, they may both act as the reservoir (storing the pathogen within their body) and the source (releasing the pathogen into the environment). In other cases, the reservoir and the source may be different. For example, a mosquito may act as the source of an infection by transmitting the pathogen to a human through its bite, while the human may act as the reservoir for the pathogen.

There are several ways this source can infect people:

Common source outbreaks

Outbreaks, where all (or most) cases were infected by the same source, are called common source outbreaks.

Point source outbreaks

Common source outbreaks where the source has infected cases at one particular geographical location and during a short period of time. In such situations, the source is located 'at a single point in time and place'. These outbreaks have a typical bell-shaped epidemic curve that increases sharply, peaks, and then declines sharply, reflecting the normal distribution of the incubation period of the causative agent in humans. For this reason, the epidemic curve of a point source outbreak can help identify the moment of transmission (i.e., when all cases have been exposed to the source).

Continuing common source outbreaks

Outbreaks where all (or most) cases have been infected by the same source over a prolonged period of time. The shape of the epidemic curve does not increase that sharply, it does not peak, yet reaches a plateau sustained over time until the source is removed.

Propagated outbreaks

Outbreaks of communicable infectious disease (i.e. can be transmitted from person to person) for which there is no single, common source. The causative agent is propagated within the population through human contact patterns. The shape of the epidemic curve in propagated outbreaks can vary and depends on the contact pattern and the proportion of susceptible individuals.