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