Biologists presume that all organisms possess viruses and that new viruses are always escaping from organisms' genomes and evolving. The Chinese practiced variolation for thousands of years. In the 1790s, Edward Jenner noted that milkmaids who had contracted cowpox were immune to smallpox. He developed a vaccination technique by deliberately infecting people with cowpox virus. Interestingly, the explanation of how vaccination worked did not come for over a hundred years.
Vaccination is one of several protocols whereby immunity is conferred.
The word 'vacca' in Latin means cow. By the end of the 19th century, some workers noted that the infectious units of tobacco mosaic disease passed through a porcelain filter, thus documenting that the causative agent is smaller than a bacterium. In 1933, Stanley purified an extract of the causative agent of tobacco mosaic disease as crystals which were determined to still be able to cause disease. Hence, viruses are not living organisms, but are characterized as chemicals. In mammals, immunity to viruses results from the production of antibodies to chemical components of the outer coating of virions.