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Cowpox is a skin disease caused by a virus known as the cowpox virus. The pox is related to the vaccinia virus and got its name from the distribution of the disease when dairymaids touched the udders of infected cows. The ailment manifests itself in the form of red blisters, and is transmitted by touch from infected animals to humans. Cowpox is similar to but much milder than the highly contagious and often deadly smallpox disease. It resembles mild smallpox, and was the basis of the first smallpox vaccines. When the patient recovers from cowpox, the person is immune to smallpox.
The cowpox virus was used to perform the first successful vaccination against smallpox, which is caused by the related Variola virus. Therefore, the word "vaccination" — first used by Edward Jenner (an English physician) in 1796 — has the Latin root vaccinus meaning of or from cows. World Health Organization in 1980 announced that smallpox was the first disease that had been eradicated world wide by a program of vaccination. Despite the eradication of smallpox in the past century, other orthopoxviruses, such as monkeypox virus, vaccinia virus in Brazil, and cowpox virus (CPXV) in Europe, still infect humans. CPXV has been restricted to the Old World with wild rodents as its natural reservoir. Human CPXV infections are commonly described in relation to contact with diseased domestic cats, sometimes directly from rats or domesticated house mice. Human infections usually remain localized and self-limiting, but can become fatal in immunosuppressed patients. In 2010 a laboratory worker contracted cowpox. 
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In the years from 1770 to 1790, at least six people who had contact with a cow had tested independently the possibility of using the cowpox vaccine as an immunization for smallpox in humans for the first time; among them were the English farmer Benjamin Jesty, in Dorset, in 1774 and the German teacher Peter Plett in 1791. Jesty inoculated his wife and two young sons and thus spared them probable death by smallpox which was raging in the area in which they lived. His patients who had contracted and recovered from the similar but milder cowpox (mainly milkmaids), seemed to be immune not only to further cases of cowpox, but also to smallpox. By scratching the fluid from cowpox lesions into the skin of healthy individuals, he was able to immunize those people against smallpox. Reportedly, farmers and people working regularly with cattle and horses were often spared during smallpox outbreaks. More and more, an investigation conducted towards 1790 by the British Army showed that horse-mounted troops were less infected by smallpox than infantry, and this due to a major exposure to the similar horse pox virus (Variola equina). By the early 19th century, more than 100,000 persons in Great Britain had been vaccinated. The arm-to-arm method was also used to distribute Jenner's vaccine throughout the Spanish Empire. Spanish king Charles IV's daughter had been stricken with smallpox in 1798, and after she recovered, he arranged for the rest of his family to be vaccinated. In 1803, the king, convinced of the benefits of the vaccine, ordered his personal physician Francis Xavier de Balmis, to deliver it to the Spanish dominions in North and South America. To maintain the vaccine in an available state during the voyage, the physician recruited from the orphanages of Spain 22 young boys, aged three to nine years, who had never had cowpox or smallpox before. During the trip across the Atlantic, de Balmis vaccinated the orphans in a living chain. Two children were vaccinated immediately before departure, and when cowpox pustules had appeared on their arms, material from these lesions was used to vaccinate two more children.
Jesty did not publicise his findings, however, and credit was assumed by Jenner, who performed his first inoculation 22 years later and publicised his findings. It is said that Jenner made this discovery by himself possibly without knowing previous accounts 20 years earlier. Although Jesty could have been the first to discover it, Jenner let everyone know and understand it, thus taking credit for it.
The virus is found in Europe, and mainly in the UK. Human cases today are very rare and most often contracted from domestic cats. The virus is not commonly found in cattle; the reservoir hosts for the virus are woodland rodents, particularly voles. From these rodents, domestic cats contract the virus. Symptoms in cats include lesions on the face, neck, forelimbs, and paws, and less commonly upper respiratory tract infections. Symptoms of infection with cowpox virus in humans are localized, pustular lesions generally found on the hands and limited to the site of introduction. The incubation period is 9 to 10 days. The virus is prevalent in late summer and autumn.
Kinepox is an alternate term for the smallpox vaccine used in early 19th-century America. Popularized by Jenner in the late 1790s, kinepox was a far safer method for inoculating people against smallpox than the previous method, variolation, which had a 3% fatality rate.
In a famous letter to Meriwether Lewis in 1803, Thomas Jefferson instructed the Lewis and Clark expedition to "carry with you some matter of the kine-pox; inform those of them with whom you may be, of its efficacy as a preservative from the smallpox; & encourage them in the use of it..." Jefferson had developed an interest in protecting Native Americans from smallpox, having been aware of epidemics along the Missouri River during the previous century. A year before his special instructions to Lewis, Jefferson had persuaded a visiting delegation of North American Indian Chieftains to be vaccinated with kinepox during the winter of 1801-2. Unfortunately, Lewis never got the opportunity to use kinepox during the pair's expedition, as it had become inadvertently inactive — a common occurrence in a time before vaccines were stabilized with preservatives such as glycerol or kept at refrigeration temperatures, cowpox is not serious or dangerous and no deaths occurred from it.
Historical use 
Cowpox was the original vaccine of sorts for smallpox. After infection with the disease, the body (usually) gains the ability to recognize the similar smallpox virus from its antigens, so is able to fight the smallpox disease much more efficiently. The vaccinia virus now used for smallpox vaccination is sufficiently different from the cowpox virus found in the wild as to be considered a separate virus. The virus contains 186 thousand base pairs of DNA, which contains the information for about 187 genes. This makes cowpox one of the most complicated viruses known. Some 100 of these genes give instructions for key parts of the human immune system, giving a clue as to why the closely related smallpox is so lethal.
As cowpox virus was tested to invent a smallpox vaccine, prevention against cowpox is by vaccination against smallpox.
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