Sex, strains and virulence.

When are populations of infectious agents likely to evolve into distinct strains? Are they likely to differ in their virulence? Will genetically distinct strains or clones remain stable long enough to be useful as epidemiological markers? Sexual recombination can break down the genetic associations that define a strain structure, but if sex is rare or inbreeding is common, can strains persist? In this paper Ian Hastings and Bruce Wedgwood-Oppenheim show how some simple population genetic theory may provide a basis for addressing these questions.