ABSTRACT
A virus is essentially a collection of genes enclosed within a protein coat and often, in about half of the known animal viruses, is surrounded by a membrane. Many of these viruses are sufficiently stable in the environment, or in vectors, to ensure their transmissibility to other hosts. It is important to understand how and where viruses persist in the body, how they are controlled, and to what extent they change their properties during persistence. The generally persistent RNA virus infections are accompanied by some replication, with the result that mutations must continue to accumulate throughout persistence. In order to succeed in evolutionary terms, viruses must be capable of replication, persistence within a host, and transmission to other individual hosts of the same or different species. Viruses which are spread through the environment in large amounts are likely to infect animals of different species.
