Virus
A virus is a microscopic infectious agent that can replicate only inside the living cells of an organism. Viruses infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea. The study of viruses is known as virology, a sub-discipline of microbiology.
Structurally, viruses are composed of genetic material, either DNA or RNA, surrounded by a protective coat of protein called a capsid. Some viruses also have an additional lipid envelope derived from the host cell membrane. The genetic material encodes the information necessary for the virus to replicate and produce new virus particles, or virions, within a host cell.
Viruses are unique in that they are not considered living organisms. They lack the cellular machinery necessary for metabolism and cannot reproduce independently. Instead, they must hijack the cellular machinery of a host organism to replicate. This dependency on host cells is a defining characteristic of viruses and distinguishes them from other microorganisms like bacteria and fungi.
The life cycle of a virus typically involves several stages: attachment, penetration, uncoating, replication, assembly, and release. During attachment, the virus binds to specific receptors on the surface of a host cell. Once attached, the virus penetrates the cell, often by endocytosis or membrane fusion. The viral capsid is then removed in a process called uncoating, releasing the viral genetic material into the host cell. The host’s cellular machinery is then co-opted to replicate the viral genome and synthesize viral proteins. These components are assembled into new virions, which are released from the host cell to infect new cells.
Viruses can cause a wide range of diseases in humans, animals, and plants. In humans, viral infections can range from mild illnesses like the common cold to severe diseases such as HIV/AIDS, influenza, and COVID-19. Some viruses can also lead to chronic infections or contribute to the development of cancer.
The impact of viruses extends beyond health, influencing ecological and evolutionary processes. They play a role in regulating populations of organisms and driving genetic diversity through horizontal gene transfer. In biotechnology, viruses are used as tools for gene therapy and vaccine development.
In summary, viruses are complex entities that straddle the boundary between living and non-living. Their ability to infect and replicate within host cells makes them both fascinating subjects of study and significant challenges in medicine and public health. Understanding viruses is crucial for developing strategies to prevent and treat viral infections, as well as harnessing their potential in scientific and medical applications.
| Virus | |
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| SARS-CoV-2, a member of the subfamily Orthocoronavirinae | |
Virus classification 
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| (unranked): | Virus |
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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. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. 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. The study of viruses is known as virology, a subspeciality of microbiology.
When infected, a host cell is often forced to rapidly produce thousands of copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent viral particles, or virions, consisting of (i) genetic material, i.e., long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts; (ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids. 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.
The origins of viruses in the evolutionary history of life are still unclear. Some viruses may have evolved from plasmids, which are pieces of DNA that can move between cells. Other viruses may have evolved from bacteria. 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. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life" and as replicators.
Viruses spread in many ways. 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. Many viruses spread in the air by coughing and sneezing, including influenza viruses, SARS-CoV-2, chickenpox, smallpox, and measles. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by hand-to-mouth contact or in food or water. The infectious dose of norovirus required to produce infection in humans is fewer than 100 particles. 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. Several classes of antiviral drugs have been developed and around 100 individual antiviral drugs or combinations of them are used worldwide.