InTrodUctIOn

Viruses are the smallest forms of life on earth, so dependant on higher organisms that they cannot reproduce outside of cells. Yet, human contact with these submicroscopic parasites-- whose name comes from the Latin for poison-- has led to widespread and sometimes disastrous consequences( the smallpox epidemics that devastated the Amerindians, the 1918 influenza A pandemic that claimed 20 to 40 millions lives, the current Aids epidemic).

In 1892, in an expirement to isolate the cause of tobacco mosaic diseases.Dmitri Iwanowski filtered thesap of diseased tobacco plants through a poecelain filter that was designed to retain bacteria.He expected to find the microbe trapped in the filter; instead, he found that the infectious agent had passed through minute pores of the filter. When he injected healthy plants with the filtered fluid, they contacted tobacco mosaic diseases.

Iwanowski still believed the infectious agent to be a bacterium to be a bacterium that was small enough to pass through the filter but later, other scientisrs, led by Martinus Beijerinck , observed that the behaviour of the agent was different from that of bacteria. In the early 1900s, a distincton was finally made between viruses and bacteria, the filterable agents that cause tobacco mosaic diseases and many other diseases. ( Virus in Latin means poison.) In 1935, an American chemist, Wendell M.Stanley, isolated the tobacco mosaic viruse (genus Tobamovirus), making it possible for the first time to carry out chemical and structural studies on a purified virus. At about the same time, the invention of electron microscope made it possible to see vieuses for the first time.

Since the isolation of the first virus in 1892, scientists have made tremendous advances by asking one question at a time and building on the answers. What are viruses? What do they look like? How are they transmitted from host to host? How do they interact with the immune system, and how can they be fought? In the last thrity years especially, with the advent of molecular biology, virologists have developed richly detailed characterizations of viral structure, replication, and genetics--information that revolutionized our understanding of genetics processes and our ability to combat viral infections. Particularly striking was the discovery that the extreme volatility and rapid evolution of some viruses results from the storage of their genetic codes in RNA rather than DNA - a phenomenon not found in any other living organisms.

Over the span of several generations, the word virus has had two meanings. A century ago, "virus" reffered to a vague poison associated with the disease and death. Physicians would suggest that the air was filled virus, or that a virus was in the blood. Louis Pasteur and his contemporaries wrote freely about the "cholera virus" and the "rabies virus"; and in the vernacular of the times, a bacterium was the virus of tuberculosis.

The modern notion of a virus is dramatically different. Viruses are recognized as particles of nucleic acid and protein, often with a covering membrane. They replicate in living cells and cause a number of important diseases such as genital herpes, influenza, hepatitis, and infectious mononucleousis. Virus consider vary considerably in size, shape, and chemical composition , and the methods used in their cultivation and the detection are completely different than for other microorganisms. The term "rabies virus" is still a common in microbiology , but with a vastly different meaning than in Pasteur's time.