Epidemiology

Viral epidemiology is the branch of medical science that deals with the transmission and control of virus infections in humans. Transmission of viruses can be vertical, that is from mother to child, or horizontal, which means from person to person. Examples of vertical transmission include hepatitis B virus and HIV where the baby is born already infected with the virus. Another, more rare, example is the varicella zoster virus, which although causing relatively mild infections in humans, can be fatal to the foetus and newly born baby. Horizontal transmission is the most common mechanism of spread of viruses in populations. Transmission can be exchange of blood by sexual activity, e.g. HIV, hepatitis B and hepatitis C; by mouth by exchange of saliva, e.g. Epstein-Barr virus, or from contaminated food or water, e.g. norovirus; by breathing in viruses in the form of aerosols, e.g. influenza virus; and by insect vectors such as mosquitoes, e.g. dengue. The rate or speed of transmission of viral infections depends on factors that include population density, the number of susceptible individuals, (i.e. those who are not immune), the quality of health care and the weather.

Epidemiology is used to break the chain of infection in populations during outbreaks of viral diseases. Control measures are used that are based on knowledge of how the virus is transmitted. It is important to find the source, or sources, of the outbreak and to identify the virus. Once the virus has been identified, the chain of transmission can sometimes be broken by vaccines. When vaccines are not available sanitation and disinfection can be effective. Often infected people are isolated from the rest of the community and those that have been exposed to the virus placed in quarantine. To control the outbreak of foot and mouth disease in cattle in Britain in 2001, thousands of cattle were slaughtered. Most viral infections of humans and other animals have incubation periods during which the infection causes no signs or symptoms.Incubation periods for viral diseases range from a few days to weeks but are known for most infections. Somewhat overlapping, but mainly following the incubation period, there is a period of communicability; a time when an infected individual or animal is contagious and can infect another person or animal. This too is known for many viral infections and knowledge the length of both periods is important in the control of outbreaks. When outbreaks cause an unusually high proportion of cases in a population, community or region they are called epidemics. If outbreaks spread worldwide they are called pandemics.