Medmicro Chapter 48 Epidemiology and Evolution Frank Fenner General Concepts Aspects of Viral Epidemiology Viruses must enter cells to multiply. Portals of Entry and Exit Portals of entry and exit in horizontal transmission include all body surfaces, or the blood stream, by arthropod bite. Vertical transmission may occur in the ovum, via the placenta, during birth, or in the colostrum or milk. The mode of exit is not necessarily the same as the portal of entry. Some human viral infections are acquired from an animal source, which may be via an arthropod in which the virus may multiply.
Many infections have a seasonal incidence.
Many viruses undergo recombination, and those with segmented genomes may undergo reassortment. Evolutionary changes are due to the interplay of genetic variability and natural selection, often during the transmission phase. Epidemiology and evolution are linked because epidemiologic mechanisms of transfer largely determine the natural selection component of viral evolution. Since viruses multiply only within cells, the epidemiology of viral diseases does not involve multiplication in food, water, or soil. However, some viruses that infect man may multiply and persist in other animals, such as arthropods, rodents and bats.