noun, plural: viruses
A submicroscopic infectious agent that is unable to grow or reproduce outside a host cell. It is non-cellular but consisting of a core of DNA or RNA surrounded by a protein coat.
A virus requires a living cell for it to grow and reproduce similar to an obligate intracellular parasite. Many viruses can cause disease in host organisms but not all can cause disease. The first known virus, tobacco mosaic virus, was discovered by Martinus Beijerinck in 1899.
Viruses may be classified based on their type of genome (Baltimore classification):
- Class I : dsDNA viruses, i.e. with double stranded DNA genome
- Class II : ssDNA viruses, i.e. with single stranded DNA genome
- Class III : dsRNA viruses, i.e. with a double stranded RNA genome
- Class IV : (+)ssRNA viruses, i.e. with a positive sense, single stranded RNA genome, the genome itself acting as mRNA
- Class V : (-)ssRNA viruses, i.e with a negative sense, single stranded RNA genome used as a template for mRNA synthesis
- Class VI : ssRNA-RT viruses, i.e. with a positive sense, single stranded RNA genome but with a DNA intermediate not only in replication but also in mRNA synthesis
- Class VII : dsDNA-RT viruses