| A virus (from the Latin noun virus, | | | | Sarah Nelmes, a milkmaid who had |
| meaning toxin or poison) is a | | | | previously caught cowpox and was |
| microscopic particle (ranging in size | | | | subsequently found to be immune to |
| from 20 - 300 nm) that can infect the | | | | smallpox, a similar, but devastating |
| cells of a biological organism. Viruses | | | | virus. Jenner developed the first |
| can replicate themselves only by | | | | vaccine based on these findings; after |
| infecting a host cell. They therefore | | | | lengthy (but successful) vaccination |
| cannot reproduce on their own. At the | | | | campaigns the World Health Organization |
| most basic level, viruses consist of | | | | (WHO) certified the eradication of |
| genetic material contained within a | | | | smallpox in 1979. |
| protective protein coat called a capsid. | | | | In the late 19th century Charles |
| They infect a wide variety of organisms: | | | | Chamberland developed a porcelain filter |
| both eukaryotes (animals, plants, | | | | capable of isolating viruses. This |
| protists, and fungi) and prokaryotes | | | | filter was used to study the first |
| (bacteria and archaea). A virus that | | | | documented virus, tobacco mosaic virus. |
| infects bacteria is known as a | | | | Shortly afterwards, Dimitri Ivanovski |
| bacteriophage, often shortened to phage. | | | | published experiments showing that |
| The study of viruses is known as | | | | crushed leaf extracts of infected |
| virology and people who study viruses | | | | tobacco plants were still infectious |
| are known as virologists. Viruses cause | | | | even after filtering the bacteria from |
| several serious human diseases, such as | | | | the solution. At about the same time, |
| AIDS, influenza and rabies. Therapy is | | | | several others documented filterable |
| difficult for viral diseases as | | | | disease-causing agents, with several |
| antibiotics have no effect on viruses | | | | independent experiments showing that |
| and few antiviral drugs are known. The | | | | viruses were different from bacteria, |
| best way to prevent viral diseases is | | | | yet they could also cause disease in |
| with a vaccine, which produces immunity. | | | | living organisms. These experiments |
| It has been argued extensively whether | | | | showed that viruses are orders of |
| viruses are living organisms. Most | | | | magnitudes smaller than bacteria. The |
| virologists consider them non-living, as | | | | term virus was coined by the Dutch |
| they do not meet all the criteria of the | | | | microbiologist Martinus Beijerinck. |
| generally accepted definition of life. | | | | In the early 20th century, Frederick |
| They do not respond to changes in the | | | | Twort discovered that bacteria could be |
| enviroment, which is a trait among | | | | attacked by viruses. Felix d'Herelle, |
| living organisms. | | | | working independently, showed that a |
| Discovery | | | | preparation of viruses caused areas of |
| Viral diseases such as rabies, yellow | | | | cellular death on thin cell cultures |
| fever and smallpox have affected humans | | | | spread on agar. Counting the dead areas |
| for many centuries. There is | | | | allowed him to estimate the original |
| hieroglyphical evidence of polio in the | | | | number of viruses in the suspension. The |
| ancient Egyptian empire,[1] however, the | | | | invention of Electron microscopy |
| cause of these diseases was unknown at | | | | provided the first look at viruses. In |
| the time. In 1717, Mary Montagu, the | | | | 1935 Wendell Stanley crystallised the |
| wife of an English ambassador to the | | | | tobacco mosaic virus and found it to be |
| Ottoman Empire, observed local women | | | | mostly protein. A short time later the |
| inoculating their children against | | | | virus was separated into protein and |
| smallpox. In the late 18th century, | | | | nucleic acid parts. |
| Edward Jenner observed and studied Miss | | | | |