The principle obstacle to human dominion over the rain forests is still the rich variety of parasites lying… — William H. McNeill

When variations could be so extravagantly successful, displacement of one humanoid population by another even more effective group of hunters must have occurred frequently. Survival was more likely for the more formidable in battle as well as for the more efficient in the hunt.

Decisive breakthroughs in military medical administration came just after the turn of the twentieth century. Until then, in even the best-managed armies, disease was always a far more lethal factor than enemy action, even during active campaigns.

Looked at from the point of view of other organisms, humankind resembles an acute epidemic disease, whose occasional lapses into less virulent forms of behavior have never yet sufficed to permit any really stable, chronic relationship to establish itself.