Because viruses are too small to be filtered out of a solution, and too small to see, 19th centuries scientists called them by the Latin name for poison: virus.
A virus is typically between 20 and 300 nanometres across, a protein coat surrounding nucleic acid. It needs to get into a cell to reproduce itself.
Viruses are small packets of genetic material in a protein coat. They can only reproduce inside a living cell, which they destroy in the process.
A virus can be called life because it can reproduce and mutate, or non-life because it needs to invade a cell to be able to reproduce. Take your choice.
Because viruses can only reproduce inside a living cell, they are often classed as non-living, but on the other hand, they contain genetic material.
Some viruses specialize in attacking bacteria. These are known as bacteriophages, and some bacteriophages have been used to treat bacterial infections.
In 1915, Frederick Twort suggested that bacteriophages (as we now know them) were viruses which attack bacteria - these were later referred to as 'phages'.
In 1917, Felix Hubert D'Herelle, independently of Frederick Twort, also discovered the same effect, and it was he who called it a bacteriophage.
In 1945, Max Delbrück and Salvador Luria organized the first phage course at Cold Spring Harbor Laboratory which would be taught for 26 consecutive years.
In 1953, André Lwoff, working with bacteriophage lambda, found that phage viruses are capable of inserting their genome into the host genome.
In 1955, Seymour Benzer began fine-structure genetic mapping a phage, a process that would take five years. He concluded that a gene has many mutable sites.
In 1981, the first reports of AIDS began to surface. Symptoms had been noted earlier, especially an increase in Kaposi's Sarcoma, but now AIDS was a condition.
AIDS is caused by HIV, the human immunodeficiency virus. There are some people who argue that there are other causes, but the medical evidence points at HIV.
HIV can be passed from mother to child, and in the absence of any of the alleged 'lifestyle causes' can develop into AIDS, and kill the child.
AIDS and HIV do not kill people: the virus takes away the normal immune response, leaving people open to attack by diseases that would usually be controlled.
AIDS is caused by HIV, but in a very real sense, AIDS can be said to be caused by poverty, because poor people are more likely to be infected by HIV.
The Durban declaration of 2000 was drafted to counter a set of mischievous and ill-informed claims from 'AIDS sceptics' that HIV was unrelated to AIDS.
In 1898, Martinus Beijerinck used filtering trials to show that tobacco mosaic disease is caused by something smaller than a bacteria and called it a virus.
In 1910, Peyton Rous showed that viruses play a role in some cancers when he discovered the Rous Sarcoma Virus. He gained a Nobel Prize for this in 1966.
In 1937, Sir Frederick Charles Bawden discovered that the tobacco mosaic virus contains RNA, the first virus found to contain RNA as the genetic material.
In 1935, Wendell Meredith Stanley was the first researcher to purify and crystallize a virus, the tobacco mosaic virus, for which he gained a 1946 Nobel Prize.
In 1955, Fraenkel-Conrat and Williams separated tobacco mosaic virus (TMV) nucleic acid from its protein coat and found that both were necessary for infection.
In 1960 Heinz Fraenkel-Conrat announced the complete sequencing of the 158 amino acids which make up the protein coat of the tobacco mosaic virus.
Smallpox is a viral disease, and like many viral diseases, it can be prevented with a suitable vaccine that prepares the immune system to attack the virus.
In 1717 Lady Mary Wortley Montagu had two of her children variolated against smallpox. The practice continued until Edward Jenner developed vaccination.
In 1776, George Washington had his troops inoculated against smallpox, using the pre-vaccination treatment called variolation, which was common in his time.
Variolation was a procedure that usually gave people a mild dose of smallpox, but occasionally, it killed. In any case, it was a lesser risk than doing nothing.
In 1796, Edward Jenner was ethical when he attempted to apply the standard inoculation with smallpox (variolation) on a boy who had previously been vaccinated.
The point of was that variolation usually caused a mild form of smallpox, but was known to give immunity. Jenner's vaccination offered risk-free immunity.
In 1977, there were no cases of smallpox known, anywhere in the world, as it had, by then, been wiped out in the wild. Laboratory stocks still exist.
An arbovirus is a virus spread by blood-sucking arthropods: it is short for 'arthropod-borne virus'. A number of serious human diseases are from arboviruses.
Rabies is a viral disease spread by animals. It attacks the nervous system producing the classic symptoms of 'madness' associated with the disease.