Examples of common human diseases caused by viruses include the common cold, influenza, chickenpox and cold sores. Many serious diseases such as ebola, AIDS, avian influenza and SARS are caused by viruses. The relative ability of viruses to cause disease is described in terms of virulence. Other diseases are under investigation as to whether they too have a virus as the causative agent, such as the possible connection between human herpes virus six (HHV6) and neurological diseases such as multiple sclerosis and chronic fatigue syndrome. There is controversy over whether the borna virus, previously thought to cause neurological diseases in horses, could be responsible for psychiatric illnesses in humans.[107]
Viruses have different mechanisms by which they produce disease in an organism, which largely depends on the viral species. Mechanisms at the cellular level primarily include cell lysis, the breaking open and subsequent death of the cell. In multicellular organisms, if enough cells die the whole organism will start to suffer the effects. Although viruses cause disruption of healthy homeostasis, resulting in disease, they may exist relatively harmlessly within an organism. An example would include the ability of the herpes simplex virus, which causes cold sores, to remain in a dormant state within the human body. This is called latency[108] and is a characteristic of the all herpes viruses including the Epstein-Barr virus, which causes glandular fever, and the varicella zoster virus, which causes chickenpox. Most people have been infected with at least one of these types of herpes virus.[109] However, these latent viruses might sometimes be beneficial, as the presence of the virus can increase immunity against bacterial pathogens, such as Yersinia pestis.[110] On the other hand, latent chickenpox infections return in later life as the disease called shingles.
Some viruses can cause life-long or chronic infections, where the viruses continue to replicate in the body despite the host's defence mechanisms.[111] This is common in hepatitis B virus and hepatitis C virus infections. People chronically infected are known as carriers, as they serve as reservoirs of infectious virus.[112] In populations with a high proportion of carriers, the disease is said to be endemic.[113] In contrast to acute lytic viral infections this persistence implies compatible interactions with the host organism. Persistent viruses may even broaden the evolutionary potential of host species.[114]
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.[115] 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.[116] 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),[117] the quality of health care and the weather.[118]
Epidemiology is used to break the chain of infection in populations during outbreaks of viral diseases.[119] 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.[120] To control the outbreak of foot and mouth disease in cattle in Britain in 2001, thousands of cattle were slaughtered.[121] Most viral infections of humans and other animals have incubation periods during which the infection causes no signs or symptoms.[122] Incubation periods for viral diseases range from a few days to weeks but are known for most infections.[123] 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.[124] This too is known for many viral infections and knowledge the length of both periods is important in the control of outbreaks.[125] 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.
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