INFECTIONS: THEY’RE HERE
Some infections will not peter out on their own even with all the infrastructure money can buy: infections transmitted by sex. But almost all the venereal diseases that are important in one geographic region have already spread globally. This global spread happened long ago because sexually transmitted pathogens tend to have prolonged periods of infectiousness, and people the world over are having sex. People infected with sexually transmitted organisms can carry them across oceans and mountains and around the world, whenever the people themselves can make such voyages. Could an early warning system work against these truly dangerous globe-trotting pathogens? Some experts imagine massive investments in surveillance and interdiction, but that proposal sounds disturbingly like the efforts directed against drug smuggling. Surely it would be an act of desperation. Surveillance and interdiction might provide an effective barrier against pathogens that pose little threat of spread, such as the Ebola virus, but that would be like building two cages around a dangerous animal when one cage would do. How could such containment work for the more serious threats, such as a new sexually transmitted pathogen, a new AIDS virus? Such pathogens are generally asymptomatic or cryptically symptomatic during much of their infectiousness. Are we going to test each of the millions of international travelers for venereal infection and make them wait with immigration services until the results of their tests are available? The complete failure to control the spread of HIV into new countries during the last decade in spite of the clearest signs of the impending disaster reveals how hopeless such an approach would be. Experts on AIDS are currently arguing about whether HIV arose naturally through transfer from chimpanzees to humans, or whether medical activities, such as vaccination or reuse of contaminated syringes, played a critical role. If HIV arose naturally, it serves as a warning of the kinds of things that could happen naturally in the future. Yet even if HIV did arise from a chimpanzee virus without any help from medicine, there is a bright side: few new AIDS-like pathogens are likely to invade from some isolated group of humans because there are hardly any isolated groups of humans left on the planet. The recognition of the exaggerated threat from far-off places does come with a few caveats, however. One caveat involves the historical recognition that germs have entered human populations primarily from two sources. The first source is domesticated animals. Unless we start introducing pathogens from domesticated animals in new ways—such as through the transplanting of their organs—the threat from domesticated animals probably holds few new surprises. The second source is other primates. Our biological machinery has diverged evolutionarily from that of other primates more recently than it has from more distantly related species. If a germ enters a human from another primate, especially from another ape, the barriers to setting up shop will be relatively low. At the time of the transfer, the germ’s characteristics will have evolved to take advantage of that other primate; but because that primate is more similar biochemically to humans than, say, lions, we can expect a greater proportion of these transfers to take hold. The process is much like the differential settlement of North America. Immigrants did not settle the continent randomly but rather settled in places where the climate and topography was similar to their homeland. Scandinavians tended to settle in Minnesota, and Germans in Wisconsin. Those from the British Isles settled more heavily along the Atlantic coast. To some extent these differences depended on what was available, but that cannot be the whole story. Who would be more likely to say that Minnesota winters are not so harsh—an immigrant from England or an immigrant from Norway? Germs, of course, do not have the option of forethought; still, one can ask whether a germ that enters a human from a chimpanzee will find life in humans more harsh than would a germ from a lion. It’s no surprise that although African cats and African primates have retroviruses, we humans have gotten our retroviruses, the HIVs and the HTLVs, from other primates rather than from cats. It is not just the ability to infect and grow in humans that is important. It is also the ability to get out of a human and into the next human. Many germs have passed the first test but failed the second. The track record indicates that here, too, germs from other primates are more likely to become successfully established in humans. The vector-borne diseases that can perpetuate themselves indefinitely in humans—the malarias, yellow fever, dengue—have come disproportionately from other primates. Other vertebrates have made an occasional contribution—sleeping sickness from ungulates, Chagas’ disease from opossums, plague from rats—but these contributions are vastly less than would be expected if the contributions were proportional to the numbers of these other species. The greatest pool of closely related primates is in Africa, which also has its share of other potential sources, such as ungulates and rodents. We can therefore expect the dribble of germs that will enter and become established in humans to do so in sub-Saharan Africa more often than in other geographic areas. Rodents do seem to be sending pathogens our way, but many of these rodent pathogens do not persist in humans. Two examples familiar to North Americans are the bacterium that causes Lyme disease and the hantaviruses that cause Four Corners disease. Rodents are probably common sources for such diseases because they live in such close association with us and in such great numbers. Occasionally they cause terrible damage, as occurred with the black plague, but even that disease fizzled out in the human population. That the threat from far-off places is only minor does not mean, however, that we have nothing to fear but fear itself.*33\225\2*
