The Evolution and Eradication of Infectious Diseases

BRIEF PROPOSALS THE EVOLUTION AND ERADICATION OF INFECTIOUS DISEASES T. AIDAN COCKBURN, M.D.* Many years ago I asked a distinguished scientist a question: "Where did measles come from?" He replied, "Measles has always been there." I then asked, "How long is always?" and was told that "always is always." This was not a very satisfying conversation, but it did illuminate brightly a very common form of thinking among physicians with regard to the origin ofthe diseases they handle. A disease like measles does not consist merely ofthe measles virus, or ofthe human host, or ofthe environment in which both live; it is a combination ofall three. Unless the proper virus, the proper host, and the proper environment are brought together at the same time, there will be no measles. Since man did not exist ten million years ago, obviouslymeasles could not have existed at that time. More than 50 per cent ofthe content ofmy book The Evolution and Eradication ofInfectious Diseases1 is devoted to a series oftheories which try to show how and when infectious diseases in general and those ofman in particular evolved through the years since the origin oflife on earth. The basic assumption is that all parasites are descended from organisms that were once free-living. Parasites can spread in two main ways. In "horizontal" spread, parasites are transmitted from animals to animals or species to species in a short period oftime, as during a measles epidemic or in the zoonoses when an infection is transmitted from an animal to a human being. "Vertical" spread takes place over millions ofyears. A population ofhost animals will break up into smaller groups and eventually evolve into new species. The parasites to which the original population was host will, ofcourse, accompany the animals to which they were attached, so that in the course of time a series of new species will appear which all carry the same parasites. For example, the ancestral host of all the primates must have had a series ofparasites which have been handed down, little changed, to all the primate radiation. Thus man, apes, and monkeys are all infected with lice, malaria, intestinal protozoa, pinworms, etc., which vary only slightly according to the families ofthe host. Modern theory holds that man evolved over some millions of years in Africa. Many of his original infections must therefore have evolved on that continent. However, as man spread around the world some two hundred thousand years ago, some ofthe para- * Assistant Commissioner of Health, Office of Board ofHealth, City Hall, Cincinnati 2, Ohio. 1 Johns Hopkins Press, 1963. 498 BriefProposals Perspectives in Biology and Medicine · Summer 1964 sites which required intermediate hosts found only in Africa would have been unable to survive on the new continents. In addition, migrant early man became exposed to new zoonoses in the fresh lands he occupied. In this way a pattern ofhost-parasite relationships would have been set up which survived until about ten thousand years ago. At that time two major revolutions occurred: man discovered how to domesticate animals, and he invented agriculture. This in turn led to a vast increase in population and the founding of large cities. Urbanization permitted, for the first time, the permanent establishment of organisms that exist solely by rapid transfer from one man to another—like measles, smallpox, and the common cold. Since the organisms of these "crowd" diseases often resemble those ofdomestic animals, the assumption has beenmade that most were derived from this source. The evolution ofdiseases in the past is intimately related to the actions we take against them today. Within recent years a new concept of "eradication" is replacing the "control " ofinfectious disease as a desirable target. By eradication is meant the complete wiping -out ofa pathogen causing a disease so that it cannot come back and no further measures against it are required. Obviously, the problem oferadication is closely tied up with the evolution ofthe pathogen, for, ifan organism has an identical relationship with that ofsome animal host, then eradicating it from the human host is not enough. Ifthe human parasitehas close relationship with another in some animal host, then the possibility exists that the organism may evolve again from these...