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A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. [1] Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. [2] [3] Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity.
Gamma phage, an example of virus particles (visualised by electron microscopy) Virology is the scientific study of biological viruses.It is a subfield of microbiology that focuses on their detection, structure, classification and evolution, their methods of infection and exploitation of host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they ...
A virus with this "viral envelope" uses it—along with specific receptors—to enter a new host cell. Viruses vary in shape from the simple helical and icosahedral to more complex structures. Viruses range in size from 20 to 300 nanometres; it would take 33,000 to 500,000 of them, side by side, to stretch to 1 centimetre (0.4 in).
The virus was finally isolated by Max Theiler (1899–1972) in 1932 who went on to develop a successful vaccine. [10] By 1928 enough was known about viruses to enable the publication of Filterable Viruses, a collection of essays covering all known viruses edited by Thomas Milton Rivers (1888–1962).
Viruses enter host cells using a variety of mechanisms, including the endocytic and non-endocytic routes. [4] They can also fuse at the plasma membrane and can spread within the host via fusion or cell-cell fusion. [5] Viruses attach to proteins on the host cell surface known as cellular receptors or attachment factors to aid entry. [6]
Viral evolution is a subfield of evolutionary biology and virology that is specifically concerned with the evolution of viruses. [1] [2] Viruses have short generation times, and many—in particular RNA viruses—have relatively high mutation rates (on the order of one point mutation or more per genome per round of replication).
Virus tropism refers to the virus' preferential site of replication in discrete cell types within an organ. In most cases, tropism is determined by the ability of the viral surface proteins to fuse or bind to surface receptors of specific target cells to establish infection.
Viruses infect all life forms; therefore the bacterial, plant, and animal cells and material in the gut also carry viruses. [6] When viruses cause harm by infecting the cells in the body, a symptomatic disease may develop. Contrary to common belief, harmful viruses may be in the minority, compared to benign viruses in the human body.