The Red Queen Race

Hosts and parasites are trapped in an evolutionary arms race. The host evolves a defense; the parasite evolves a way around it; the host counters again. Biologist Leigh Van Valen named this the Red Queen hypothesis, after the character who runs flat out just to stay in the same spot.

You carry the evidence. Some of the fastest-changing genes in the human genome are immune genes, churning constantly because any fixed defense would be cracked within a few generations of parasite pressure.