Stay Away From Viruses

A virus (from the Latin virus meaning toxin or poison) is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host cell. Virus diseases inflict a heavy illness and economic burden on humans and animals and can devastate agricultural crops. Each viral particle, or virion, consists of genetic material, DNA or RNA, within a protective protein coat called a capsid. Their shape varies from simple helical and icosahedral (round) forms to more complex structures with tails or an envelope. Viruses infect all cellular forms of life, and they are separated into animal, plant and bacterial viruses. It has been argued whether viruses are living organisms. Some consider them non-living as they do not meet all the criteria of the definition of life. For example, unlike most organisms, viruses do not have cells. However, viruses have genes and evolve by natural selection. They have been described as organisms at the edge of life. Viral infections usually results in disease and, in animal hosts, an immune response. Often, the virus is completely eliminated by the immune system. In comparison with antibiotics (which have no effect on viruses) antiviral drugs are generally more specific and more expensive. For some serious viral diseases vaccines, which can produce lifelong immunity, have been developed.

Viral diseases such as rabies, yellow fever and smallpox have affected humans for many centuries. There is hieroglyphical evidence of polio in the ancient Egyptian empire, though the cause of these diseases was unknown at the time. In 1717, Mary Montagu, the wife of an English ambassador to the Ottoman Empire, observed local women inoculating their children against smallpox. In the late 18th century, Edward Jenner observed and studied Miss Sarah Nelmes, a milkmaid who had previously caught cowpox and was subsequently found to be immune to smallpox, a similar, but devastating virus. Jenner developed the first vaccine based on these findings; after lengthy (but successful) vaccination campaigns the World Health Organization (WHO) certified the eradication of smallpox in 1979.

In the late 19th century Charles Chamberland developed a porcelain filter with pores small enough to filter bacteria, yet retain all viable viruses. Dimitri Ivanovski used this filter to study tobacco mosaic virus. He published experiments showing that crushed leaf extracts of infected tobacco plants were still infectious after filtering through such filters. At about the same time, several others documented filterable disease-causing agents, with several independent experiments showing that viruses were different from bacteria, yet they could also cause disease in living organisms. These experiments showed that viruses are orders of magnitudes smaller than bacteria. The term virus was coined by the Dutch microbiologist Martinus Beijerinck who showed, using methods based on the work of Ivanovski, tobacco mosaic disease is caused by something smaller than a bacterium. He developed with the term "contagium vivum fluidum" which means “soluble living germ” as first the idea of the virus.

In the early 20th century, Frederick Twort discovered that bacteria could be attacked by viruses. Felix d'Herelle, working independently, showed that a preparation of viruses caused areas of cellular death on thin cell cultures spread on agar. Counting the dead areas allowed him to estimate the original number of viruses in the suspension. The invention of Electron microscopy provided the first look at viruses. In 1935 Wendell Stanley crystallised the tobacco mosaic virus and found it to be mostly protein. A short time later the virus was separated into protein and nucleic acid parts. In 1939, Max Delbrück and E.L. Ellis demonstrated that, in contrast to cellular organisms, bacteriophage reproduce in "one step", rather than exponentially.

A major problem for early virologists was the inability to propagate viruses on sterile culture media, as is done with cellular microorganisms. This limitation required medical virologists to infect living animals with infectious material, which is dangerous. The first breakthrough came in 1931, when Ernest William Goodpasture demonstrated the growth of influenza and several other viruses in fertile chicken eggs. However, many viruses would not grow in chicken eggs, and a more flexible technique was needed for propagation of viruses. The solution came in 1949 when John Franklin Enders, Thomas H. Weller and Frederick Chapman Robbins together developed a technique to grow polio virus in cultures of living animal cells. Their methods have since been extended and applied to the growth of many viruses and other infectious agents that do not grow on sterile culture media.

info : wikipedia