There are two types of parasite allele - call them P1 and P2.

P1 is adapted to penetrate host organisms with the H1 allele

But can't parasitize hosts with the H2 allele.

P2 penetrates organisms with the H2 allele

But is unable to penetrate organisms with the H1 allele.

Let's look at the frequencies of these alleles.

When parasite genotype P2 is more common, host genotype H1 is favored and will increase in frequency.

Then, as host H1 becomes more frequent, the fitness of parasite P1 increases and its frequency rises.

When parasite genotype P1 is more common, host organisms with the H2 allele are favored, and numbers of H1 hosts thus start to decrease.

Thus the gene frequency oscillates cyclically.