Flint et al. Principles of Virology (ASM), Chapter 1 and 2
Wagner &Hewlett. Basic Virology (Blackwell). Chapter 10
Historical Perspective
Many viruses have co-evolved with mammals and other animals over long periods of time. Examples of such viruses are herpesviruses, which have been traced back to fish and birds, as well as mammals. It is thought that herpesviruses have existed for two hundred million years or longer, and that they have infected humans since the early times of our speciation. Other viruses have entered human populations only recently, due to changes in agriculture (use of domestic animals), population dynamics (urbanization), migration of populations, commerce and changes in the environment. Examples of these agents include measles virus and HIV-1.
Vaccines
Attempts to control smallpox have existed for almost one thousand years. The early method was called variolation, and involved the inoculation of healthy individuals with material from a smallpox pustule, by scratching of the arm. This crude method was effective, but was accompanied by serious side effects including disseminated skin lesions and even death in about 1% of cases. However, this was preferable to the very high fatality rate of the natural infection (25% or so, and more in children).The concept of vaccination (vacca = cow) arose in the 1790s, when Jenner made the observation that milk maids exposed to cowpox were protected against smallpox. Jenner showed that the deliberate inoculation of a boy with cowpox virus resulted in protection from smallpox. The cowpox vaccine was propagated for many years in humans, before being grown in animals, but at some point the cowpox virus unknowingly became replaced by the virus now known as vaccinia. Vaccinia virus has been found in “cowpox” vaccines dating from the 1870s, and it is not clear how the virus arose – although it may have arisen in horses.
The derivation of deliberately attenuated vaccines did not occur until the work of Pasteur in the 1880s, who serially passaged rabies virus in rabbits in order to derive a weakened (attenuated) strain that still elicited protective immunity but which failed to cause disease.
Discovery of viruses
In the 1890s, scientists discovered that the agent which caused tobacco mosaic disease was a filterable agent smaller than bacteria. By the early 1900s, additional viruses had been identified, including viruses which caused tumors in chickens (e.g., the Rous sarcoma virus) as well as yellow fever virus (the first human virus to be discovered, in 1901).Definitive Properties of Viruses
Perhaps the defining single feature of viruses is that they are obligate intracellular molecular parasites. A more complete list of the defining properties of viruses is as follows:
- Viruses are obligate intracellular molecular parasites, which are very small and infectious.
- The virus genome is composed either of DNA or RNA.
- The virus genome directs the synthesis of virion components within an appropriate host cell.
- Progeny virus particles are produced by the assembly of newly made viral components.
- Progeny virus particles spread infection to new cells.
Size of viruses
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