Viruses survive in nature only if they are able to pass from one host to another, whether of the same or another species. Viral epidemiology is the study of the factors that determine the frequency and distribution of viral diseases in an animal population. In the broadest sense, epidemiology may be viewed as a part of population biology, involving not only environmental factors but also genetic factors in both the virus and the host. The terms incidence and prevalence are used to describe quantitative aspects of the occurrence of infections in populations. The incidence of infection is defined as the proportion of a population contracting that infection during a specified period, whereas prevalence refers to the proportion infected at a particular point in time. The comparisons of incidence and prevalence at different times and places are made by relating the appropriate numerator to a denominator that may be as general as the total population of the animal species concerned or may be specified as the susceptible population at risk. A disease is said to be enzootic when there are continuous chains of transmission in the region involved; epizootics are peaks in disease incidence. The size of the peak required to constitute an epizootic is arbitrary and is related to the background enzootic rate, the morbidity, and the anxiety that the disease arouses.
Epidemiology of Viral Infections