“In view of the truly extraordinary record of the past few centuries, no one can say for sure that new and unexpected breakthroughs will not occur, expanding the range of the possible beyond anything easily conceived of now. Birth control may in time catch up with death control. Something like a stable balance between human numbers and resources may then begin to define itself. But for the present and short-range future, it remains obvious that humanity is in course of one of the most massive and extraordinary ecological upheavals the planet has ever known. Not stability but a sequence of sharp alterations and abrupt oscillations in existing balances between microparasitism and macroparasitism can therefore be expected in the near future as in the recent past.
“In any effort to understand what lies ahead, as much as what lies behind, the role of infectious disease cannot properly be left out of consideration. Ingenuity, knowledge, and organization alter but cannot cancel humanity’s vulnerability to invasion by parasitic forms of life. Infections disease which antedated the emergence of humankind will last as long as humanity itself, and will surely remain, as it has been hitherto, one of the fundamental parameters and determinants of human history.”