History of virology

The word virus appeared in 1599 and originally meant "venom".[5] A very early form of vaccination known as variolation was developed several thousand years ago in China. It involved the application of materials from smallpox sufferers in order to immunize others. In 1717 Lady Mary Wortley Montagu observed the practice in Istanbul and attempted to popularize it in Britain, but encountered considerable resistance. In 1796 Edward Jenner developed a much safer method, using cowpox to successfully immunize a young boy against smallpox, and this practice was widely adopted. Vaccinations against other viral diseases followed, including the successful rabies vaccination by Louis Pasteur in 1886. The nature of viruses however was not clear to these researchers. Martinus Beijerinck In 1892 Dimitri Ivanovski showed that a disease of tobacco plants, tobacco mosaic disease, could be transmitted by extracts that were passed through filters fine enough to exclude even the smallest known bacteria. In 1898 Martinus Beijerinck repeated Iwanowski's work but went further and passed the "filterable agent" from plant to plant, found the action undiminished, and concluded it infectiousŚreplicating in the hostŚand thus not a mere toxin. He called it contagium vivum fluidum.[6] The question of whether the agent was a "living fluid" or a particle was however still open. In 1903 it was suggested for the first time that transduction by viruses might cause cancer. In 1908 Bang and Ellerman showed that a filterable virus could transmit chicken leukemia, data largely ignored till the 1930s when leukemia became regarded as cancerous.[7] In 1911 Peyton Rous reported the transmission of chicken sarcoma, a solid tumor, with a virus, and thus Rous became "father of tumor virology".[7] The virus was later called Rous sarcoma virus 1 and understood to be a retrovirus. Several other cancer-causing retroviruses have since been described. The existence of viruses that infect bacteria (bacteriophages) was first recognized by Frederick Twort in 1911, and, independently, by Felix d'Herelle in 1917. As bacteria could be grown easily in culture, this led to an explosion of virology research. The cause of the devastating Spanish flu pandemic of 1918 was initially unclear. In late 1918, French scientists showed that a "filter-passing virus" could transmi

the disease to people and animals, fulfilling Koch's postulates.[8] In 1926 it was shown that scarlet fever is caused by a bacterium that is infected by a certain bacteriophage. While plant viruses and bacteriophages can be grown comparatively easily, animal viruses normally require a living host animal, which complicates their study immensely. In 1931 it was shown that influenza virus could be grown in fertilized chicken eggs, a method that is still used today to produce vaccines. In 1937, Max Theiler managed to grow the yellow fever virus in chicken eggs and produced a vaccine from an attenuated virus strain; this vaccine saved millions of lives and is still being used today. Max Delbruck, an important investigator in the area of bacteriophages, described the basic "life cycle" of a virus in 1937: rather than "growing", a virus particle is assembled from its constituent pieces in one step; eventually it leaves the host cell to infect other cells. The Hershey-Chase experiment in 1952 showed that only DNA and not protein enters a bacterial cell upon infection with bacteriophage T2. Transduction of bacteria by bacteriophages was first described in the same year. In 1949 John F. Enders, Thomas Weller and Frederick Robbins reported growth of poliovirus in cultured human embryonal cells, the first significant example of an animal virus grown outside of animals or chicken eggs. This work aided Jonas Salk in deriving a polio vaccine from deactivated polio viruses; this vaccine was shown to be effective in 1955. The first virus that could be crystalized and whose structure could therefore be elucidated in detail was tobacco mosaic virus (TMV), the virus that had been studied earlier by Ivanovski and Beijerink. In 1935, Wendell Stanley achieved its crystallization for electron microscopy and showed that it remains active even after crystallization. Clear X-ray diffraction pictures of the crystallized virus were obtained by Bernal and Fankuchen in 1941. Based on such pictures, Rosalind Franklin proposed the full structure of the tobacco mosaic virus in 1955. Also in 1955, Heinz Fraenkel-Conrat and Robley Williams showed that purified TMV RNA and its capsid (coat) protein can self-assemble into functional virions, suggesting that this assembly mechanism is also used within the host cell, as Delbruck had proposed earlier.