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- Virology, the study of viruses, includes many aspects: the molecular biology of virus replication; the structure of viruses; the interactions of viruses and hosts and the diseases they cause in those hosts; the evolution and history of viruses and viral diseases; virus epidemiology, the ecological niche occupied by viruses and how they spread from victim to victim; and the prevention of viral disease by vaccination, drugs, or other methods.
www.ncbi.nlm.nih.gov/pmc/articles/PMC7149408/
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- Molecular Biology
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- Viruses
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- Molecular Biology
- Overview
- Definition
- Host range and distribution
A virus is an infectious agent of small size and simple composition that can multiply only in living cells of animals, plants, or bacteria.
What are viruses made of?
A virus particle is made up of genetic material housed inside a protein shell, or capsid. The genetic material, or genome, of a virus may consist of single-stranded or double-stranded DNA or RNA and may be linear or circular in form.
What size are viruses?
Most viruses vary in diameter from 20 nanometres (nm; 0.0000008 inch) to 250–400 nm. The largest viruses measure about 500 nm in diameter and are about 700–1,000 nm in length.
Are all viruses spherical in shape?
Viruses occupy a special taxonomic position: they are not plants, animals, or prokaryotic bacteria (single-cell organisms without defined nuclei), and they are generally placed in their own kingdom. In fact, viruses should not even be considered organisms, in the strictest sense, because they are not free-living—i.e., they cannot reproduce and carry on metabolic processes without a host cell.
All true viruses contain nucleic acid—either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid)—and protein. The nucleic acid encodes the genetic information unique for each virus. The infective, extracellular (outside the cell) form of a virus is called the virion. It contains at least one unique protein synthesized by specific genes in the nucleic acid of that virus. In virtually all viruses, at least one of these proteins forms a shell (called a capsid) around the nucleic acid. Certain viruses also have other proteins internal to the capsid; some of these proteins act as enzymes, often during the synthesis of viral nucleic acids. Viroids (meaning “viruslike”) are disease-causing organisms that contain only nucleic acid and have no structural proteins. Other viruslike particles called prions are composed primarily of a protein tightly complexed with a small nucleic acid molecule. Prions are very resistant to inactivation and appear to cause degenerative brain disease in mammals, including humans.
Viruses are quintessential parasites; they depend on the host cell for almost all of their life-sustaining functions. Unlike true organisms, viruses cannot synthesize proteins, because they lack ribosomes (cell organelles) for the translation of viral messenger RNA (mRNA; a complementary copy of the nucleic acid of the nucleus that associates with ribosomes and directs protein synthesis) into proteins. Viruses must use the ribosomes of their host cells to translate viral mRNA into viral proteins.
Viruses are also energy parasites; unlike cells, they cannot generate or store energy in the form of adenosine triphosphate (ATP). The virus derives energy, as well as all other metabolic functions, from the host cell. The invading virus uses the nucleotides and amino acids of the host cell to synthesize its nucleic acids and proteins, respectively. Some viruses use the lipids and sugar chains of the host cell to form their membranes and glycoproteins (proteins linked to short polymers consisting of several sugars).
The true infectious part of any virus is its nucleic acid, either DNA or RNA but never both. In many viruses, but not all, the nucleic acid alone, stripped of its capsid, can infect (transfect) cells, although considerably less efficiently than can the intact virions.
The virion capsid has three functions: (1) to protect the viral nucleic acid from digestion by certain enzymes (nucleases), (2) to furnish sites on its surface that recognize and attach (adsorb) the virion to receptors on the surface of the host cell, and, in some viruses, (3) to provide proteins that form part of a specialized component that enables the virion to penetrate through the cell surface membrane or, in special cases, to inject the infectious nucleic acid into the interior of the host cell.
Logic originally dictated that viruses be identified on the basis of the host they infect. This is justified in many cases but not in others, and the host range and distribution of viruses are only one criterion for their classification. It is still traditional to divide viruses into three categories: those that infect animals, plants, or bacteria.
Virtually all plant viruses are transmitted by insects or other organisms (vectors) that feed on plants. The hosts of animal viruses vary from protozoans (single-celled animal organisms) to humans. Many viruses infect either invertebrate animals or vertebrates, and some infect both. Certain viruses that cause serious diseases of animals and humans are carried by arthropods. These vector-borne viruses multiply in both the invertebrate vector and the vertebrate host.
Airborne transmission is traditionally defined as involving the inhalation of infectious aerosols or “droplet nuclei” smaller than 5 μm and mainly at a distance of >1 to 2 m away from the infected individual, and such transmission has been thought to be relevant only for “unusual” diseases.
- Chia C. Wang, Chia C. Wang, Kimberly A. Prather, Josué Sznitman, Jose L. Jimenez, Jose L. Jimenez, S...
- 2021
May 7, 2024 · WHO has revised its criteria for how diseases can spread through the air, based on expert advice and evidence from covid-19. The new definition does not rely on droplet size or distance, but on whether droplets are inhaled or deposited on mucus membranes.
Learn about the structure, classification, and types of viruses, including the coronavirus that causes COVID-19. Find out how viruses infect hosts, how they are different from bacteria, and why soap is effective against them.
Jul 27, 2012 · Viruses spread by an oral–fecal route are disseminated by ingestion of contaminated food or water. Infection begins in the gut, and it may or may not spread to other organs. Many of these viruses cause gastroenteritis. Virus is excreted in feces or urine to continue the cycle.
Mar 16, 2020 · The coronavirus disease 19 (COVID-19) is a highly transmittable and pathogenic viral infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which emerged in Wuhan, China and spread around the world.
