A virus is a small, infectious, obligate intracellular parasite, capable of replicating itself in a host cell. Virions are formed by de novo assembly from newly synthesized components: the genome and a number of copies of at least one viral protein (capsid, or coat protein). Then the virions exit the cell and enter new cells, thereby beginning a new infectious cycle.

Different viruses employ different replication strategies and encode different kinds of proteins. For instance, many viruses encode their own RNA-polymerases or DNA-polymerases. Others rely upon cellular replication machinery, but in this case they encode at least one regulatory protein required for replication. Viruses of yet another type, so-called satellites, can only replicate with the help of other viruses' proteins.

Viroids constitute a separate group of subviral agents. These are small, circular, single-stranded RNAs which autonomously replicate in plant cells, but neither encode proteins nor form virions.

A viral genome consists of either single-stranded or double-stranded DNA or RNA in either linear or circular form, and can comprise one or more segments.