A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea.[1] Since Dmitri Ivanovsky's 1892 article describing a non-bacterial pathogen infecting tobacco plants, and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898,[2] about 5,000 viruses have been described in detail,[3] although there are millions of different types.[4] Viruses are found in almost every ecosystem on Earth and are the most abundant type of biological entity.[5][6] The study of viruses is known as virology, a sub-speciality of microbiology. source: http://en.wikipedia.org/wiki/Virus