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A virus is an extremely small infectious agent. A virus consists of a protein coating over a strand of RNA or DNA, which replicates itself by taking over a host cell of another life form, implanting its genetic information into it, and causing the cell to produce more viruses. In the type of reproduction used by virulent viruses, the lytic cycle, the host cell bursts, dispersing the new viruses. Other viruses do not destroy the host cell, but multiply by budding. With these methods of reproduction, viruses play a part in maintaining genetic diversity through transfer of genetic material between cells.
Like all things related to medicine there is a lot of woo regarding viruses. Most commonly said woo questions whether viruses are linked to certain diseases, how effective vaccines are in preventing virus infection, or even whether viruses can cause infection at all.
There is some debate as to whether viruses are living organisms or not. While they do contain genetic material (i.e. the DNA or RNA), and evolve, like any other living organism, they also contain no cell structures (things like a cytoplasm, cell wall, nucleus, et cetera), lack senses or a metabolism, and cannot reproduce independently. The existence of giant viruses that can be "attacked" in the same way as a cell by smaller viruses, has also led to some controversy regarding the definition of life, and whether viruses fit it.
There are also other virus-like infectious agents like viroids, which are just strands of RNA without the protein coating and cause plant diseases, and prions, which are just protein particles and can cause, for example, degenerative disorders of the central nervous system like mad cow disease.
While the ability of viruses to infect cells is generally bad for health, there are some possible interesting applications in science and technology, like using viruses to reduce cancer.
Damaged or weakened viruses are used in vaccination; they are processed by the immune system, which can then produce antibodies against the full-strength virus if later infected. Mutation of the virus may render past vaccines ineffective against new strains; viruses which frequently mutate in ways that alter their surface antigens such as influenza, the common cold and HIV have proven very difficult to effectively counter.
Since they are invisible and insidious, they play into many very basic human fears. However, viral disease can, for the most part, be contained by modern sanitary measures such as isolation, decontamination and proper sanitary conditions. Gigantic death tolls are generally associated with ignorance about how disease is spread, improper application of procedures, or superstition and denialism overcoming medical consensus.
Antibiotics are ineffective in the treatment of viral infections, as they are designed instead to target bacteria. In fact, the use of antibiotics for viruses can even lead to the propagation of antibiotic resistant bacteria.
There are thousands of virus types of which we are aware and have studied, and thousands, or even millions more which we have yet to discover, with there being an estimated 320 000 new types of virus to be discovered just for mammals. Here are a few notable ones.
Smallpox is caused by a virus known as Variola Major, a member of the family poxviridae or poxviruses. Smallpox is a disease that only affects humans and likely first arose in agricultural Mesopotamian populations, at around 5000 BCE. It is a very infectious disease that can be spread by breathing in infected droplets of saliva While the average mortality rate is only about 30%, this only led to the disease being even more infectious, which, in turn, led to the disease killing several million people, with 300-500 million deaths observed in the 20th century alone. It is notable in that it was the first infectious disease to be eradicated, due to a worldwide vaccination programme, with the last naturally occurring case being observed in Somalia, 1977.
The human immunodeficiency virus (HIV) is known to cause the condition known as acquired immunodeficiency syndrome (AIDS) which it achieves by damaging, destroying, or impeding the functioning of various T-helper cells in the immune system, particularly CD4+ cells. Not everyone believes this, however, which leads to HIV denialism, which is strongly connected with alternative medicine practitioners and vaccine denialism.
The influenza virus is best known for its astounding variety of mutations, as well as routinely causing pandemics during the 20th century (most notably, the Spanish flu killed between 50 and 100 million, more than all deaths from World War I, which occurred concomitantly). It mutates very quickly, rendering (current) influenza vaccines useless within a year, but since there remains a need for them, vaccines are reformulated every year.
Gastroenteritis is sometimes called "stomach flu," but is unrelated to the influenza virus.
The Ebola virus is known for its very short incubation period and its awesome lethality, exhibiting a death rate that can reach as high as 90%. Fortunately, it is only infectious through the transmission of bodily fluids — this means that it is relatively difficult to transmit, and is, despite its high lethality, unlikely to cause serious worldwide "apocalyptic" epidemics.
Viruses and evolution
- The HIV virus, for example, is, on average, only ~145 nanometres in size, as explained here
- Rybicki, EP. The classification of organisms at the edge of life, or problems with virus systematics. S Afr J Sci. 1990;86:182–186.
- And if that doesn't seem like a lot, consider all of the other forms of life on Earth that could serve to be viral hosts.
- As fewer patients died before transmitting the disease
- In the mid 20th century, smallpox vaccination scars on the skin over the deltoid muscle were a common sight, more the rule than the exception.
- These changes happen often enough that your immune system can’t recognize the flu virus from year to year.