The Red Queen hypothesis: hosts and pathogens are locked in a perpetual evolutionary arms race. Resistance alleles spread when pathogens are common; virulence alleles spread when resistance is rare. The result is endless oscillating coevolution.
Δp_H = s · p_H(1−p_H)(p_P − 1/2) | Δp_P = s · p_P(1−p_P)(p_H − 1/2) | Period ~ 1/(s·mutation)