WHERE ARE THEY COMING FROM AIDS

It certainly is a melting pot for pathogens, just as it is still a melting pot for humans. But is the gathering of foreign pathogens in the melting pot of North America the real problem? Are the most important threats to North America coming from distant places? There certainly is no shortage of pathogens crossing international borders. A new scare surfaces almost every year. In the summer and early fall of 1999 it was the West Nile virus, which killed seven people, some horses, and several thousand crows along the eastern seaboard of the United States. Then the outbreak fizzled. Even when a full-fledged vector-borne pathogen like the dengue virus or the malaria protozoan gets into the United States, it peters out on its own. If these masters of
vector-borne transmission cannot make a go of it in humans, what chance does a West Nile virus have? It is out of its element in humans. It cannot be transmitted from human to mosquito to human even in the best of situations. It attracts attention mainly because it is exotic.
People tend to overestimate the risk of accidents that are rare but sensational—the airplane crashes—relative to the much more dangerous but commonplace accidents—the car crashes. So too we tend to overestimate the danger from the unusual when it is lethal. But statistics can correct our mistakes. People who look at the actual calculations of the probability of dying per mile traveled in an airplane can see that it is vastly less than the probability of dying per mile traveled in a car and adjust their worries accordingly. The same thing can be done with infectious diseases, though people have to spend a bit more effort in educating themselves about the diseases with known microbial causes, the microbes that cause them, and the diseases that may be caused by unknown microbes. If people do not do this extra legwork, the government and its control agencies will continue to go down the path of least resistance. If the public is more frightened about Ebola, hantavirus, Legionnaires’ disease, and West Nile virus than about the probable infectious causes of chronic diseases like cancer, stroke, heart attack, multiple sclerosis, and Alzheimer’s, then the public will get overfunding of low-level infectious threats and underfunding of high-level infectious threats.
Because the AIDS virus originated in sub-Saharan Africa, the AIDS pandemic has fixed worried attention on what might be out there ready to emerge from steamy African jungles or urban Asian swarms, just a plane ride away from the hubs of Euro-American civilization. Many leading experts have called for interventions to block this emergence. Jonathan Mann, the first head of the World Health Organization’s AIDS program, called for a worldwide early warning system “to detect quickly the eruption of new diseases or the unusual spread of old diseases.” It sounds like a good idea on the face of it, but how effective would it be? The history of disease emergence provides some clues.
Even superficial digging unearths many examples of diseases that immigrated into prosperous Western countries from poorer ones: yellow fever, repeated outbreaks of influenza, Asiatic cholera, black plague, malaria, and of course, AIDS. But there is something telling in this list, or any other representative list one might construct. With the exception of AIDS, the diseases first made the trip to Euro-American populations many decades or even centuries ago.
Their global mobility depended on two critical factors: the group size of the traveling humans and the speed of travel. Human travel needed to be sufficiently rapid to allow disease organisms to be maintained within the group of travelers. Meeting this requirement is more tricky than it sounds because the human immune system is so good at subduing pathogens. Within a few weeks pathogens are usually eradicated from the body or so decimated that they have little chance of transmission. Pathogens that had mastered the trick of prolonging infectiousness were probably transported between the Mediterranean and south and central Asia from the earliest days of intercontinental trade. Pathogens that had not mastered the trick would have to wait for improvements in transportation technology to move them throughout the civilized world.
But they would not have to wait for jumbo jets. Caravans and horses were sufficient for diseases such as the black plague to make their way from central Asia to southern Europe nearly two thousand years ago. For other diseases, the movement of big boats permitted travel over expansive tracts of humanless terrain. In the early nineteenth century Asiatic cholera finally made the passage from its south Asian homeland to Europe and then did so repeatedly, causing waves of turbulence in Europe and America until water supplies were protected. Boats filled with dozens of people may have been necessary to permit its passage because the cholera organism can rarely withstand a person’s defenses for more than a couple of weeks. Yellow fever similarly was barred from the Americas until ships could act as microcosms, complete with tiny pools of freshwater in which the mosquito vectors could breed. The African mosquito Aedes aegypti colonized the boats and eventually the urban centers of the New World, thus cycling the virus in people both during and after the transatlantic voyage. Jet travel is overkill, by about five hundred miles per hour.
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