In epidemiology, an infection is said to be endemic in a population when that infection is maintained in the population without the need for external inputs. For example, chickenpox is endemic in the UK, but malaria is not. Every year, there are a few cases of malaria acquired in the UK, but these do not lead to sustained transmission in the population due to the lack of a suitable vector (mosquitoes of the genus Anopheles). Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems (Last 2001). ... Chicken pox, also spelled chickenpox, is a common childhood disease caused by the varicella-zoster virus (VZV), also known as human herpes virus 3 (HHV-3), one of the eight herpesviruses known to affect humans. ... Red blood cell infected with Malaria (Italian: bad air; formerly called ague or marsh fever in English) is an infectious disease which in humans causes about 350-500 million infections and over 1 million deaths annually, mainly in the tropics and sub-Saharan Africa. ... Traditionally in medicine, a vector is an organism that does not cause disease itself but which spreads infection by conveying pathogens from one host to another. ... Some Species Anopheles beklemishevi Anopheles coustani Anopheles crypticus Anopheles farauti Anopheles forattinii Anopheles funestus Anopheles gambiae Anopheles grabhamii Anopheles hailarensis Anopheles halophylus Anopheles hyrcanus Anopheles kosiensis Anopheles maculipennis Anopheles minimus Anopheles moucheti Anopheles nili Anopheles ovengensis Anopheles pampanae Anopheles peytoni Anopheles quadrimaculatus Anopheles rennellensis Anopheles rivulorum Anopheles triannulatus Anopheles is...

For an infection to be endemic, each person who becomes infected with the disease must pass it on to exactly one other person (on average). Assuming a completely susceptible population, that means that the basic reproduction number (R₀) of the infection must equal 1. In a population with some immune individuals, the basic reproduction number multiplied by the proportion of susceptible individuals in the population (S) must be 1. This takes account of the probability of each individual who the disease may be transmitted to actually being susceptible to it, effectively discounting the immune sector of the population. In epidemiology, the basic reproduction number of an infection is the number of people a single infected individual will infect, in a population with no immunity to the disease. ... The immune system is the organ system that protects an organism from outside biological influences. ... In epidemiology a susceptible individual (sometimes known simply as a susceptible) is a member of a population who is at risk of becoming infected by a disease, if they are exposed to the infectious agent. ... The word probability derives from the Latin probare (to prove, or to test). ... The microorganisms (bacteria and viruses) that cause disease may be transmitted from one person to another by one or more of the following means: droplet contact - coughing or sneezing on another person direct physical contact - touching an infected person indirect contact - usually by touching a contaminated surface airborne transmission - if...

For the disease to be in an endemic steady state:

In this way, the infection neither dies out nor does the number of infected people increase exponentially but the infection is said to be in an endemic steady state. An infection that starts as an epidemic will eventually either die out (with the possibility of it resurging in a theoretically predictable cyclical manner) or reach the endemic steady state, depending on a number of factors, including the virulence of the disease and its mode of transmission. The term exponential may refer to any of several topics in mathematics: Exponential distribution Exponential function Exponential growth, exponential decay Exponential time Matrix exponential Exponential map (in differential geometry) All relate in some fashion to exponents. ... An epidemic is generally a widespread disease that affects many individuals in a population. ... In epidemiology, force of infection (denoted λ) is the rate at which susceptible individuals become infected by an infectious disease. ... The microorganisms (bacteria and viruses) that cause disease may be transmitted from one person to another by one or more of the following means: droplet contact - coughing or sneezing on another person direct physical contact - touching an infected person indirect contact - usually by touching a contaminated surface airborne transmission - if...

If a disease is in endemic steady state in a population, the relation above allows us to estimate the R₀ (an important parameter) of a particular infection. This in turn can be fed into the mathematical model of an epidemic. In epidemiology, the basic reproduction number of an infection is the number of people a single infected individual will infect, in a population with no immunity to the disease. ... A parameter is a measurement or value on which something else depends. ... It is possible to mathematically model the progress of most infectious diseases to discover the likely outcome of an epidemic or to help manage them by vaccination. ...

It should be noted that while it might be common to say that AIDS is "endemic" in Africa, this is a use of the word in its colloquial form (meaning found in an area). AIDS cases in Africa are still increasing, so the disease is not in an endemic steady state. It is more correct to call the spread of AIDS in Africa an epidemic. AIDS (Acquired Immunodeficiency Syndrome or Acquired Immune Deficiency Syndrome, sometimes written Aids) is a human disease characterized by weakening of the bodys immune system and capacity to fight infection and certain cancers. ... An epidemic is generally a widespread disease that affects many individuals in a population. ...

See also

• Epidemic
• Pandemic
• Mathematical modelling in epidemiology