Endemic (epidemiology)

For an infection that relies on person-to-person transmission, to be endemic, each person who becomes infected with the disease must pass it on to one other person on average. Assuming a completely susceptible population, that means that the basic reproduction number (R0) of the infection must equal one. In a population with some immune individuals, the basic reproduction number multiplied by the proportion of susceptible individuals in the population (S) must be one. This takes account of the probability of each individual to whom the disease may be transmitted being susceptible to it, effectively discounting the immune sector of the population. So, for a disease to be in an endemic steady state or endemic equilibrium, it holds that : R_0 \times S = 1 In this way, the infection neither dies out, nor does the number of infected people increase exponentially. 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. If a disease is in an endemic steady state in a population, the relation above allows the basic reproduction number (R0) of a particular infection to be estimated. This in turn can be fed into a mathematical model for the epidemic. Based on the reproduction number, we can define the epidemic waves, such as the first wave, second wave, etc. for COVID-19 in different regions and countries. == Misuse ==

It has been claimed that endemic COVID-19 implies that the disease severity would be mild. but in itself endemicity only means that there will be a steady, predictable number of sick people. == Related terms ==

Categories of endemic diseases ;Holoendemic: An endemic disease with an extremely high rate of infection, ;Hyperendemic: An endemic disease with a high rate of infection, This term is often used to describe the prevalence of malaria in a local area, with 10 to 50% of children showing evidence of prior infection being considered a moderate level for that disease. ;Hypoendemic: An endemic disease with a low rate of infection. Categories for non-endemic diseases ;Sporadic: A disease that appears occasionally, but, unlike endemic disease, is not always present at a steady and predictable level. ;Outbreak: An epidemic, especially one affecting a very small area, such as the people in one town or attending a single event. The 2019–2020 measles outbreaks showed a normally endemic disease causing an epidemic outbreak, primarily among unvaccinated people. ;Epidemic: A new disease that is spreading or a previously endemic disease whose infection rate is increasing significantly. Seasonal flu frequently appears as an epidemic. ;Pandemic: An epidemic affecting a very large part of the world, generally multiple countries or multiple continents. Seasonal flu is sometimes a global pandemic. == Examples ==

Infections that are usually considered endemic include: • Common cold • Lassa fever • Malaria • Polio • Rotavirus • Hepatitis C • Measles Smallpox was an endemic disease until it was eradicated through vaccination. == Etymology ==

The word endemic comes from the Greek: , , "in, within" and , , "people". == See also ==