Biology Assignment Help >> Virus, Small Infectious Agent

A virus is a small infectious agent which can replicate only inside the living cells of organisms. Mostly viruses are too small to be seen directly to a light microscope. Viruses infect all types of organisms, from plants or animals to bacteria or archaea. There are millions of different types of Viruses. These are found in almost every ecosystem on Earth or the most abundant type of biological entity. The study of viruses is called as virology, a sub-specialty of microbiology.

Virus particles which known as virions consist of two or three parts. The genetic material of viruses made from either DNA or RNA, they have long molecules that carry genetic information; a protein coat which protects these genes; or in some cases an envelope of lipids that surrounds the protein coat when they are outside a cell. The shapes of viruses from simple helical to icosahedral forms to more complex structures. The average virus is near about one one-hundredth the size of the average bacterium.

The origin of viruses in the Evolutionary History of life are not clear. Some may have evolved from plasmids – pieces of DNA which can move between cells – while others can have evolved from bacteria. In evolution, viruses take place an important means of horizontal gene transfer, which increases genetic diversity.

Viruses spread in many ways; viruses in plants which often transmitted from plant to plant by insects that feed on the sap of plants, such as aphids; viruses in animals can be carried by blood-sucking insects. These disease-bearing organisms are called as vectors. Influenza viruses are spread by sneezing or coughing. Rotavirus or Norovirus, common causes of viral gastroenteritis, are transmitted by the faecal-oral route or are passed from person to person by contact, entering the body in food or water. HIV is one of the several viruses transmitted through sexual contact or by exposure to infected blood. The range of the host cells that a virus can infect is called its "host range". This can be narrow or broad as when a virus is capable of infecting many species.

Viral infections in animals provoke an immune response which commonly eliminates the infecting virus. Immune responses can also be produced through vaccines, that confer an artificially acquired immunity to the specific viral infection. However, some viruses including those causing AIDS or viral hepatitis evade these immune responses and result in chronic infections. Antibiotics have no effect on viruses, but many antiviral drugs have been developed.

Origins of Viruses

Viruses are found where ever there is life or have probably existed since living cells first evolved. The origin of viruses is not clear because they do not form fossils, so molecular techniques are used to compare the DNA and RNA of viruses and are a useful means of investigating how they arose.

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