"Arms races . . . it is a colorful way of talking about coevolution, particularly when it is coevolution between enemies: between predator and prey, between parasite and host. Adaptations on one side call forth counter adaptations on the other side, and the counter adaptations call forth more and so on, escalating all the time.
The consequence is that the apparatus that we see gets better but the efficacy of it does not necessarily get better because the other side is getting better at the same time."