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Viruses are primitive biological infectious agents that live only in the cells of bacteria, plants, and animals. A specific virus invades and reproduces inside of a specific living cell until the cell explodes, spewing hundreds of copies; however, a virus cannot live outside of a cell. Viruses may be spherical, rod-shaped, or in the case of those that attack bacteria, like a screwdriver with clasps. Viruses are so tiny that they can only be seen by means of an electron microscope. Viruses are pseudo-life forms and do not match the commonly used definition of life. They do not have a cell structure, and must reproduce inside of another living cell. When expelled in search of a new host, they are inert until they can connect with a new host. They have characteristics of life forms while in the cells they infect, but not during times outside of an infected cell.
Every plant and animal is susceptible to viruses. Tens of thousands of viruses have been identified using electron microscopes. However, efforts to create a viral taxonomy have not succeeded for several reasons. Their origins are still obscure, and there is little in the way of a fossil record, so they are hard to place in the established domains of biological classification. Several domain names have been suggested such as Acytota. Organizations like the International Committee on Taxonomy of Viruses (ICTV) are working to find an organizing scheme. Viruses occur in plants and animals. Tobacco mosaic, a very thoroughly studied virus, is a viral disease in tobacco plants. The virus mottles the leaves and causes them to lose value. The recognition of viruses in plants began in the 1600s in the Netherlands. Tulip break virus was one of the most well-known plant viruses, which causes the petals to become ornamentally variegated.
The study of viruses took a major step when Louis Pasteur was able to use attenuated rabies viruses to make a vaccine against the disease in 1884. In 1892, Dmitri Ivanovski was able to isolate tobacco mosaic viruses, but he was not able to identify them specifically because of the limitations of microscopes. However, his work demonstrated the existence of a disease agent that was not bacterial. Marinus Beijerinck, a Dutch botanist, contributed the name virus, using a Latin word for poison. Virology, or the study of viruses, developed rapidly in the early 1900s from work done on viruses by Frederick William Twort and Felix d’Herelle. From their studies, many scientists studied bacterial viruses (phages). In 1935, Wendell M. Stanley identified protein as a part of the chemical makeup of some viruses, which enabled him to crystallize them. Since then, scientists have found that some viruses have a deoxyribonucleic acid (DNA) genome, and others have a ribonucleic acid (RNA) genome around which is a protein coating (capsid). Other viruses have lipids or proteins in their structure.
A single virus (viron) will have a DNA or RNA core surrounded by a protein coating. In some cases, there is additional protein of lipid material present. For example, all of the viral hemorrhagic fevers (arenaviruses, filoviruses, bunyaviruses, and flaviviruses) are RNA viruses covered with a fatty lipid coating. In 1911, it was discovered that viruses could cause tumors in chickens. Since then, other tumor-causing viruses have been isolated and described. In the 1980s, researchers linked some human cancers with viruses. Since then, Pap smears have become routine tests for early detection of the papilloma virus that causes cervical cancer in women. Most cancers in humans do not have a viral origin, but a DNA-type virus causes some cancers. The list of diseases caused by viruses is long. Smallpox (variola), yellow fever, mumps, measles, chicken pox, rabies, influenza, herpes, polio, and hepatitis have long plagued humans.
Antibiotics do not work with viruses once infection has been established. The ability to crystallize viruses enabled vaccines to be developed against polio (poliomyelitis) in the 1950s. Vaccinations do provide protection, but with serious limits. For some viruses no vaccine yet exists. Vaccinations against viruses have to deal with the problem that viruses mutate frequently. Influenza viruses are a global source of infection that can easily reach epidemic proportions. The Spanish Influenza Pandemic at the end of World War I killed millions of people. A source of influenza infections lies in the exchange of viruses that occurs between birds and animals, especially swine in southeast Asia. The exchanges provide opportunities for the viruses to mutate. This in turn means that new vaccines have to be developed to provide protection against the changed viral agent.
The common cold is caused by a viral infection, as are a number of other viral infections. The cold virus is highly infectious, but rarely deadly. However, some viruses that cause Ebola, Marburg, and Lassa fevers have extremely high mortality rates that can only be overcome by isolation.
Global air travel since the 1960s is making it possible for new kinds of viruses to emerge from remote places. Influenza such as SARS has been spread in this fashion. Other emerging viruses include the HIVI AIDS virus, which is now killing millions of people globally. The global spread of viruses includes those that infect animals and humans. After the West Nile virus appeared in the United States, it killed millions of birds and a few humans. Viruses spread through casual contact, the ingesting of food from infected sources (hepatitis from oysters), insect bites, or even animal bites. Airborne currents spread respiratory viruses, such as the hanta virus from rodent droppings. Others spread by body fluids. Careful sanitation helps to reduce the rate of infection.
The human immune system fights viral infections in several ways. In infections like measles with a high fever, lymphocytes use antibodies to cover the virus’s capsid, or by destroying cells infected with the virus. Mucus is used to capture and expel large amounts of respiratory viruses, while interferon, a protein-like substance made by the body, fights other viruses. Some viruses suppress the immune system and spread in the body rapidly. A few viruses move slowly. Others, like herpes, may be dormant for a long time and then have sporadic outbreaks. Viruses cost billions of dollars annually because of the damage they cause to crops and animals. However, viruses have been used to control insects and invasive species. Rabbits, a serious invasive species in Australia, have been controlled with the myxoma virus.
- J. Flint et al., Principles of Virology: Molecular Biology, Pathogenisis, and Control of Animal Viruses (ASM Press, 2003);
- Laurie Garrett, Coming Plague: Newly Emerging Diseases in a World Out of Balance (Penguin Group, 1994);
- B. McCormick, Susan Fisher-Hoch, and L.A. Horvitz, Level 4: Virus Hunters of the CDC (Barnes & Noble Books, 1999);
- H. and E.G. Strauss, Viruses and Human Disease (Elsevier Science & Technology Books, 2002);
- K. Wagner and M.J. Hewlett, Basic Virology (Blackwell Publishers, 2003).