The existence of viruses was established in 1892, when Ivanovsky discovered microscopic particles. The viruses are 10 to 100 times smaller than bacteria (their size varies from 30 nm to 300 nm). Anyway, they could be observed only in 1932, while electronic microscopy appeared.
They are " biological objects " sub-cellular endowed with genetic continuity and have a great evolutionary capacity. They consist of a nucleic acid (DNA or RNA for the retroviruses) and of a proteinic coat. The viruses are intracellular parasites. Indeed, they multiply inside living cell by using the biosynthetic machinery of the host. Hepatitis B, HIV, flu are well known viruses...
They are very much used in fundamental biology and private clinic: their DNA can be used like gene vector in genic therapy.
Infection of a bacteria by a virus