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:

1. The source: This is the source of the disease agent, which can be a person, animal, or environment.
2. The reservoir: This is the place where the disease agent can survive and multiply.
3. 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).
4. The host: This is the person or animal that becomes infected with the disease agent.
5. 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: Capability of a disease being passed on by direct contact with a diseased individual or through various transmission routes. The contagiousness of an organism is its capability of harboring or spreading the causative agent of a transmissible disease.

• Epidemic curve: An epidemic curve is a graph in which the number of new disease cases is plotted against an interval of time to describe a specific epidemic or outbreak.

• Generation time: The time that elapses between the onset of symptoms in the primary case and the onset of symptoms in the secondary case. It refers to the time it takes for the first group of patients to 'generate' the next group.

• Herd immunity: Herd immunity occurs when the vaccination of a significant portion of a population (or herd) provides a measure of protection for individuals who have not developed immunity. For contagious diseases transmitted from individual to individual, herd immunity theory proposes that chains of infection are likely to be disrupted when large numbers of a population are immune or less susceptible to the disease.

• Host: Persons or other living animals, including birds and arthropods, that offers subsistence or lodging to an infectious agent under natural conditions. 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.

• Incubation period: Period between exposure and onset of clinical symptoms.

• 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.

• Latent period: Period between exposure and infection during which there are no clinical symptoms or signs of infection in the host.

• 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: Anything that can produce disease. Typically the term is used to mean an infectious agent - a microorganism, such as a virus, bacterium, prion, or fungus that causes disease in its host.

• Reproductive rate of an infection: Number of cases one case generates on average over the course of its infectious period.

• Transmission routes: Pathway of causative agents from a source to infection of a susceptible host. The characteristic of the transmission route depends mainly on the characteristics of the causative agent and those of the host. Some microorganisms are restricted to a limited number of transition routes, whereas others can follow many different pathways to infect their hosts:

• 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 opposite to droplets, that are too large in diameter and fall to the ground relatively fast). This transmission route works particularly efficient for viruses such as the measles virus.

• Reservoir: The ‘reservoir of infection’ is the principal habitat where a specific infectious agent lives and multiplies and from which it may spread to cause disease. The reservoir is necessary for the infectious agent to survive or multiply in sufficient amounts to be transmitted to a susceptible host. In several articles, the concept of 'source' and 'reservoir' are used as synonyms, though strictly speaking, they are not. A source usually can be found at a specific time in a specific place; reservoirs are more generic 'homes' to microorganisms.

• 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 of infection is the location from which a host acquires the infection, either endogenous (i.e. originating from a person's own commensal microbial flora) or exogenous (i.e. the source is located external to the patient). This means that a source can usually be identified in a specific place and a specific time. Outbreaks can be categorized by how the causative agent (usually a microorganism) is spread within the population to reach new susceptible people. 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.