Abstract Viruses are among the most extreme parasites, being almost completely dependent upon their host for their growth and replication. Having no intermediary metabolism of their own they make use of the energy supply of the host, its production of nucleoside triphosphates for nucleic acid synthesis and amino acid for protein synthesis, and all of the machinery for protein synthesis. Within the infected cell the virus competes with the host for the supply of all these things and at the same time variants compete among themselves for survival and yield of progeny. It is the intensity of this competition that has produced the most subtle and intimate interactions between virus and host. The need to fit into a protective shell imposes tight limits on the size of the genome in most classes of virus. This means that additional functions can seldom be added simply by adding the necessary genetic information unless there is a compensating loss. But by making more efficient use of the genetic material, additional functions can be accommodated without altering the size of the genome significantly. This is seen to a remarkable degree in the small DNA viruses, where segments of the genome are translated in different reading frames to give different polypeptide sequences and where multiple alternative'splicing in messenger RNA synthesis allows the same polypeptide sequence to form part of two or even three proteins with different properties.