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A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism.

Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since the first discovery of a virus, the tobacco mosaic virus, in the 1890s, more than 16,000 of the millions of virus species have been described in detail. When infected, a host cell is often forced to rapidly produce thousands of copies of the original virus. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope and are one-hundredth the size of most bacteria. Some viruses may have evolved from plasmids, which are pieces of DNA that can move between cells. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction. Viruses are considered by some biologists to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack some key characteristics, such as cell structure, that are generally considered necessary criteria for defining life. One transmission pathway is through disease-bearing organisms known as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; and viruses in animals can be carried by blood-sucking insects. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The variety of host cells that a virus can infect is called its host range: this is narrow for viruses specialized to infect only a few species, or broad for viruses capable of infecting many. Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. Some viruses, including those that cause HIV/AIDS, HPV infection, and viral hepatitis, evade these immune responses and result in chronic infections.

The hepatitis delta virus of humans has an RNA genome similar to viroids but has a protein coat derived from hepatitis B virus and cannot produce one of its own. Although hepatitis delta virus genome may replicate independently once inside a host cell, it requires the help of hepatitis B virus to provide a protein coat so that it can be transmitted to new cells. These viruses, which are dependent on the presence of other virus species in the host cell, are called "satellites" and may represent evolutionary intermediates of viroids and viruses.

A virus has either a DNA or an RNA genome and is called a DNA virus or an RNA virus, respectively. All segments are not required to be in the same virion for the virus to be infectious, as demonstrated by brome mosaic virus and several other plant viruses.

It can be an emergent virus, one that represents a new virus, but it can also be an extant virus that has not been previously identified.

This is common in hepatitis B virus and hepatitis C virus infections.

Viruses accepted to cause human cancers include some genotypes of human papillomavirus, hepatitis B virus, hepatitis C virus, Epstein–Barr virus, Kaposi's sarcoma-associated herpesvirus and human T-lymphotropic virus.

Not all virus infections produce a protective immune response in this way.

Examples of nucleoside analogues are aciclovir for Herpes simplex virus infections and lamivudine for HIV and hepatitis B virus infections. Hepatitis C is caused by an RNA virus.

Although somewhat of a misconception, it is not the actual virus that is synthesised, but rather its DNA genome (in case of a DNA virus), or a cDNA copy of its genome (in case of RNA viruses).

Quick Facts

• Since the first discovery of a virus, the tobacco mosaic virus, in the 1890s, more than 16,000 of the millions of virus species have been described in detail.
• A virus has either a DNA or an RNA genome and is called a DNA virus or an RNA virus, respectively.
• The variety of host cells that a virus can infect is called its host range: this is narrow for viruses specialized to infect only a few species, or broad for viruses capable of infecting many.
• Viruses accepted to cause human cancers include some genotypes of human papillomavirus, hepatitis B virus, hepatitis C virus, Epstein–Barr virus, Kaposi's sarcoma-associated herpesvirus and human T-lymphotropic virus.
• When infected, a host cell is often forced to rapidly produce thousands of copies of the original virus.

Source material: Wikipedia - "Virus". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.