The Viral World

The survival of the human species is not a preordained evolutionary program. Abundant sources of genetic variation exist for viruses to learn new tricks, not necessarily confined to what happens routinely, or even frequently.

- Joshua Lederberg, Nobel Laureate

Viruses are tiny macromolecular structures that infect and incorporate their DNA or RNA into the genome of a host cell in order to make replicas of themselves. Despite their small size, viruses have been the cause of some of the world's worst epidemics. The influenza epidemic of 1918 was the result of more than 500,000 American deaths. More recently, HIV has become a leading cause of death in the United States and elsewhere. Viruses, however, are not only studied for their infectious nature but for their use in understanding basic biology. The fast reproduction rate combined with their small size make viruses a good model organism for biologists studying molecular genetics.

An electron micrograph image of the the influenza virus, the cause of the common cold. Image courtesy of Linda Stannard.
The Discovery of Viruses
The discovery of viruses begins in 1883, when the German scientist, A. Mayer, was searching for the cause of a disease that destroyed crops of tobacco. The disease, known as tobacco mosiac disease was found to spread from plant to plant despite being passed through a filter designed to remove bacteria. Another scientist found evidence that the microbe was not a bacteria because it could not be cultivated on nutrient media, nor could ethanol, which is usually toxic to bacteria, prevent the microbe of infecting more plants. Finally, in 1935 Wendell Stanley crystallized the infectious agent, thus confirming the suspicions of many scientists that there was another class of microbes exist that were distinct from bacteria.
An electron microscope image of the Tobacco Mosiac Virus (TMV). Image courtesy of Carl Wetter.
Time Line
1798 Edward Jenner developed a smallpox vaccine
1892 Dimitri I. Ivanovsk discovered filterability of TMV
1898 Friedrich Loeffler and Paul Frosch discovered filterability of foot and mouth disease
1901 Walter Reed discovered yellow fever
1909 S. Flexner and P. Lewis discovered poliomylitis
1911 J. Goldberger and J. Anderson discovered measles
1934 C. Johnson and E. Goodpasture discovered the mumps virus
1935 W. Stanley crystallized TMV
1939 G. Kausche, E. Pfankuch, and H. Ruska used electron microscopy to visualize virus
1953 J. Watson and F. Crick proposed double- helix model of DNA

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