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Defense

smallpox virus, milkmaids, simian immunodeficiency virus, chicken pox, adenovirus

Although viruses cannot be treated with antibiotics, which are effective only against bacteria, the body’s immune system has many natural defenses against virus infections. Infected cells produce interferons and other cytokines (soluble components that are largely responsible for regulating the immune response), which can signal adjacent uninfected cells to mount their defenses, enabling uninfected cells to impair virus replication. Some cytokines can cause a fever in response to viral infection; elevated body temperature retards the growth of some types of viruses. B lymphocytes produce specific antibodies that can bind and inactivate viruses. Cytotoxic T cells recognize virus-infected cells and target them for destruction. However, many viruses have evolved ways to circumvent some of these host defense mechanisms.

The development of antiviral therapies has been thwarted by the difficulty of generating drugs that can distinguish viral processes from cellular processes. Therefore, most treatments for viral diseases simply alleviate symptoms, such as fever, dehydration, and achiness. Nevertheless, antiviral drugs for influenza virus, herpesviruses, and HIV are available, and many others are in the experimental and developmental stages.

Prevention has been a more effective method of controlling virus infections. Viruses that are transmitted by insects or rodent excretions can be controlled with pesticides. Successful vaccines are currently available for poliovirus, influenza, rabies, adenovirus, rubella, yellow fever, measles, mumps, and chicken pox. Vaccines are prepared from killed (inactivated) virus, live (attenuated or weakened) virus, or isolated viral proteins (subunits). Each of these types of vaccines elicits an immune response while causing little or no disease, and there are advantages and disadvantages to each.

The principle of vaccination was discovered by British physician Edward Jenner. In 1796 Jenner observed that milkmaids in England who contracted the mild cowpox virus infection from their cows were protected from smallpox, a frequently fatal disease. In 1798 Jenner formally demonstrated that prior infection with cowpox virus protected those that he inoculated with smallpox virus (an experiment that would not meet today’s protocol standards because of its use of human subjects). In 1966 the World Health Organization (WHO) initiated a program to eradicate smallpox from the world. Because it was impossible to vaccinate the entire world population, the eradication plan was to identify cases of smallpox and then vaccinate all of the individuals in that vicinity. The last reported case of smallpox was in Somalia in October 1977. An important factor in the success of eradicating smallpox was that humans are the only host and there are no animal reservoirs for smallpox virus. The strain of poxvirus used for immunization against smallpox was called vaccinia. Introduction of the Salk (inactivated) and Sabin (live, attenuated) vaccines for poliovirus, developed in the 1950s by the American physician and epidemiologist Jonas Salk and the American virologist Albert Bruce Sabin, respectively, was responsible for a significant worldwide decline in paralytic poliomyelitis. However, polio has not been eradicated, partly because the virus can mutate and escape the host immune response. Influenza viruses mutate so rapidly that new vaccines are developed for distribution each year.

Viruses undergo very high rates of mutation (genetic alteration) largely because they lack the repair systems that cells have to safeguard against mutations. A high mutation rate enables the virus to continually adapt to new intracellular environments and to escape from the host immune response. Co-infection of the same cell with different related viruses allows for genetic reassortment (exchange of genome segments) and intramolecular recombination. Genetic alterations can alter virulence or allow viruses to gain access to new cell types or new animal hosts. Many scientists believe that HIV is derived from a closely related monkey virus, SIV (simian immunodeficiency virus), that acquired the ability to infect humans. Many of today’s emerging viruses may have similar histories.



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