Epidemiology and Control of Viral Diseases

Epidemiology is the study of the determinants, dynamics, and distribution of diseases in populations. The risk of infection and disease in an animal or animal population is determined by characteristics of the virus, the host and host population, and behavioral, environmental, and ecological factors that affect virus transmission from one host to another. Epidemiology, which is part of the science of population biology, attempts to meld these factors into a unified population-based perspective. The word epidemiology is widely used now no matter what host is concerned; the words endemic, epidemic, and pandemic are used to characterize disease states in human populations, and enzootic, epizootic, and panzootic are their equivalents in animal populations. By introducing quantitative measurements of disease trends, epidemiology has come to have a major role in advancing our understanding of the nature of diseases and in alerting and directing disease-control activities. Epidemiologic study is also effective in clarifying the role of viruses in the etiology of diseases, in understanding the interaction of viruses with environmental determinants of disease, in determining factors affecting host susceptibility, in unraveling modes of transmission, and in large-scale testing of vaccines and drugs. The case definition is a critical component of rates and proportions that should be standardized to allow comparison of disease occurrence in different populations and subpopulations. Criteria can be specified for confirmed, probable, and possible cases, depending on whether the selected criteria are pathognomonic for the viral disease of interest and whether laboratory results are available for all cases. Different case definitions can be specified at the individual animal and at the aggregate level.