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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).
Virus classification is the process of naming viruses and placing them into a taxonomic system similar to the classification systems used for cellular organisms. Viruses are classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause.
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.
The virus wraps its delicate nucleic acid with a protein shell known as the capsid, from the Latin capsa, meaning "box," in order to shield it from this hostile environment. Similar to how numerous bricks come together to form a wall, the capsid is made up of one or more distinct protein types that repeatedly repeat to form the whole capsid.
HIV-1 is the virus that was initially discovered and termed both lymphadenopathy associated virus (LAV) and human T-lymphotropic virus 3 (HTLV-III). HIV-1 is more virulent and more infective than HIV-2, [ 20 ] and is the cause of the majority of HIV infections globally.
In comparison, the genomes of the smallest known viruses capable of causing an infection by themselves are around 2,000 nucleotides long. [ 88 ] In 1976, Sanger et al. [ 89 ] presented evidence that potato spindle tuber viroid is a "single-stranded, covalently closed, circular RNA molecule, existing as a highly base-paired rod-like structure ...