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.