In host-parasite associations, there is often a gene-for-gene mechanism
of infection.  What this means is that certain parasite genes must
"match" certain host genes for the parasite to be able to infect:

					Gene 1      Gene 2
	parasite      A           B
		 host      A           b
					  ^---match   ^---No match.  No infection

	parasite      a           B
		 host      A           b
					  ^-No match  ^-No match.    No infection

	parasite      A           b
		 host      A           b
					  ^---match   ^---match.     Infection

Essentially, the fittest parasites in a population are those which can
match the genotype which is at the highest frequency in the population.
The fittest hosts, however, are those which are matched by the lowest-
frequency parasite genotype.  Since parasites generally have shorter
generation times than their hosts, parasite genotype frequencies tend
to "track" host genotype frequencies; they are able to evolve so that
their most common genotype matches the host's most common genotype.
This has the seemingly-paradoxical result that THE HOST GENOTYPE WITH
THE HIGHEST FITNESS IS AT THE LOWEST FREQUENCY IN THE POPULATION.  Of
course, that genotype therefore increases in frequency until it is at
a higher frequency.  However, since the parasites are evolving faster,
almost as soon as that host genotype has reached high frequency, THAT
GENOTYPE BECOMES THE LOWEST-FITNESS GENOTYPE.  This leads to really
neat genotypic cycling, where host genotypes bounce from common/low
fitness to rare/high fitness, and parasite genotypes follow them.

This cycling is the basis for the Red Queen theory for the maintenance
of sexual reproduction.