MicrobiologyBytes: Infection & Immunity: Epidemiology Updated: January 7, 2007 Search

Epidemiology of Infectious Disease

Epidemiology is the study of the determinants, occurence, distribution, and control of health and disease in a defined population. Keywords and/or concepts of importance:

Infection in Individuals

Infection is the term used to indicate the presence of an infectious agent in an individual or population. Infection, as opposed to passive contamination, implies colonization of the host's cells, tissues or body cavities, to the benefit of the organism.

Colonization indicates the presence of the organism without clinical or subclinical disease, whereas contamination refers to the presence of microbes on a body surface without invasion or response.

Depending upon the host-microbe interaction, infection can be silent (inapparent, asymptomatic, subclinical), or overt, causing a disease of infection.

Infecting organisms causing disease are termed pathogens. Individuals who are infected and can transmit infection to others are infectious. Silent or asymptomatic infections, such as HIV infection during the early phase, can still be infectious.

A carrier is a person who is infected with an organism but shows no evidence of disease, although disease may have been present earlier.

Acute infection implies a 'short-lived' infection, such as influenza, with or without symptoms; the period of infectivity is also short.

Chronic infection refers to a 'long-standing' condition during which the pathogen continually replicates and the patient may be persistently infectious to others, e.g., hepatitis B infection.

Latent infection refers to a persistent infection with the possibility of intermittent shedding of pathogens, e.g., varicella zoster virus causing shingles or herpes simplex virus causing cold sores or genital herpes.

Infection in Populations

Endemic infection refers to infection or disease that occurs regularly at low or moderate frequency.

Epidemics occur when there are sudden increases in frequency above endemic levels.

Pandemics are global epidemics. The size of 'outbreaks is dependant upon factors such as the ratio of susceptible to immune subjects, period of infectivity, population density, etc.

The prevalence of infection describes the number of acses in a population at a point in time, whereas the incidence refers to the number of cases arising over a defined period of time.

Secular trend refers to a change in the prevalence of infection over years. This relates to better living conditions, better hygiene, and vaccination. An example of a secular trend is the decrease in tuberculosis in the United Kingdom.

Seasonal trend refers to changes in the prevalence of infection occurring over the year, e.g., RSV outbreaks - the reason the seasonality is unclear but changes of temperature, crowding and humidity may play a role.

Seroprevalence refers to the number of individuals who have antibodies to a particular pathogen. It shows how common the pathogen is in the population. Seroprevalence is usually measured in age-bands to identify the age at which transmission is greatest.

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Spread of Infection

With respect to the spread of infection, people can be divided in to those who are susceptible, those who are infected but are not yet infectious, those who are infected and infectious, and those who are immune. Recovery from infection usually gives immunity. There are four main routes of infection:

  1. Contact. Infection can be transmitted by direct contact from skin to skin, mucosa to mucosa, skin to mucosa or mucosa to skin of the same or another person, e.g., herpes simplex causing primary oropharyngeal herpes or genital herpes. Infection may also spread through indirect contact via water and surfaces (fomites) as in communal bathing or an HIV or hepatitis B infected needle shared by several people. It may also be spread by droplets, produced by talking or sneezing, that are usually greater than 5µ, whose route of transmission is through the air. Because of their large size they are spread no further than about 1 metre. Examples include measles, and streptococcal pharyngitis.

  2. Common vehicle. A single inaminate vehicle serves to transmit the infection to multiple hosts. The most commonly involved common vehicles are food and water; but vaccines, blood can serve as a common vehicle. An animal causing rabies through biting other animals and man could also be included as a common vehicle.

  3. Air borne. Air borne infected particles are spread by droplet nuclei or dust. Droplet nuclei represent the residua of droplets that have evaporated to a size of less than 5µ in diameter. Skin squames can also serve as air-borne vehicles of infectious agents. Coughing and sneezing result in droplet nuclei and tuberculosis is an example of disease spread from man to man by this means. Psittacosis and Q fever are examples of infections spread from animals to man by the air-borne route.

  4. Vector borne. This refers to transmission by insects who may carry organisms on their surface or ingest it. Examples include malaria, dengue fever and Chaga's disease.

Portals of Entry

Man can be infected through various portals of entry including the mouth through ingestion of food, water, and milk, sucking or kissing; the respiratory tract through inhalation; the eye through direct contact; the skin through direct and indirect contact; penetration of the skin through injuries, surgery, insect and animal bites; through blood transfusion and intravenous drug abuse; transplantation; the urogenital tract through sexual intercourse and catheterisation, and across the placenta.

The portals of exit include the anus with faeces as infected material; the mouth with saliva, sputum and droplets as vehicles; the eyes through tears and exudate; body surfaces through skin, hair, crusts, and exudate; skin puncture through blood; the urogenital tract through urine, secretions, and semen, and the placenta.

Recognition of the epidemiology of infectious disease provides the means of preventing infection by public health measures,rather than preventing it by vaccination (which may be unsuccessful e.g., HIV) or treating established infection, which may be of limited effect (e.g. HIV, hepatitis B and C).

© AJC 2007.