More than 500 million years ago a set of specialized enzymes and proteins evolved to defend our primitive ancestors against assaults from the outside world. If a microbe breached the shell of some Cambrian-era fauna, the members of this early vintage immune system would stage a savage but coordinated attack on these interlopers—punching holes in cell walls, spitting out chemical toxins or simply swallowing and digesting the enemy whole. Once the invaders were dispatched, the immune battalion would start to heal damaged cells, or if the attacked cells were too badly damaged it would put them to rest.

This inflammatory immune response worked so well that many aspects of it have been preserved during the protracted aeons of evolution. We know this to be true because studies have found that we share many of the same immune genes as the lowly fruit fly—and vertebrates and invertebrates diverged from a common ancestor in excess of half a billion years ago.