"It takes all the running you can do, to keep in the same place."

-- The Red Queen, in Lewis Carroll's Through the Looking Glass

Viruses cause disease, and susceptibility to viral infection is thus selected against. Therefore, organisms have evolved and refined mechanisms of viral resistance, namely the immune system. But because viruses evolve rapidly, continued selection to adapt to a "moving target" results in the simultaneous rapid evolution of host genes that defend against viruses. This creates an ever-escalating arms race between host and virus. The evolutionary "Red Queen" hypothesis addresses such immunity proteins, which must constantly keep step with viruses on their accelerated evolutionary course.