It was first demonstrated in the late nineteenth century that human deaths from fever were
typically due to infections. As the germ theory gained ground, it replaced the old, unproven
theory that deaths from fever reflected a weak personal or even familial constitution. A new
enigma emerged at the turn of the twentieth century, when it became apparent that only a
small proportion of infected individuals die from primary infections with almost any given
microbe. Classical genetics studies gradually revealed that severe infectious diseases could …