Viruses are small, non-living parasites, which cannot replicate outside a host cell.
Viruses are grouped according to their genetic material: DNA or RNA.
DNA viruses are mostly double-stranded while RNA viruses are single-stranded.
A virus injects its genetic information into a host cell and then takes control of the cell’s machinery. This process enables the virus to make copies of its DNA or RNA and make the viral proteins inside the host cell. A virus can quickly make multiple copies of itself in one cell, release these copies to infect new host cells and make even more copies. In this way, a virus can replicate very quickly inside a host.