AIDS Pandemic Provokes Alarming Reassessments Of Infectious Disease

In 1900, infectious disease was the leading cause of mortality in the United States, accounting for at least 37 percent of deaths. By 1950, this had been mitigated to 6.8 percent and, by 1989, to 2.8 percent, with corresponding improvements in life expectancy. These numbers, of course, must be taken with a grain of salt, given the eventual preemptive role of infection in chronic illness, and many disorders whose infectious etiology is still to be recognized. Further, the relative importance of sanitation, health education, and nutrition--in contrast to medical interventions like immunization and antibiotics--has remained in contentious debate. Regardless, this achievement is trumpeted as the conquest of infectious disease--a glorious tribute, indeed, to public health and medical science. In consequence, health research since 1950 has ever more emphasized chronic constitutional afflictions like cancer, cardiovascular disease, and psychiatric disorders to the relative neglect of infection. Environmental anxieties are focused on radiation and chemical toxins rather than arthropod vectors and airborne contagion. Our complacency over the triumph of the human over the microbial has been shaken by the global HIV pandemic. In truth, the majority of the world's population has never been relieved of its load of malaria, tuberculosis, diarrheal disease, and a host of parasitoses. It has taken AIDS to provoke a new look at the threats to human health in the U.S. from infections recently thought to be conquered. These concerns were taken up last year in an Institute of Medicine task force study on emergent infections. Our report (Emerging Infections: Microbial Threats to Health in the United States, Washington, D.C., National Academy Press, 1992), the product of a broad collective effort, promptly reached unanimity that these threats urgently needed to be brought to public and policy attention. Our recommendations had to do mainly with (a) the strengthening of disease (especially virus) surveillance systems globally; (b) wider availability of existing vaccines and the removal of legal disincentives to the development of new ones; (c) acceleration of the technology to enable vaccines against new strains to be developed on a time scale commensurate with the spread of infection; (d) vector control through habitat management, pesticides, or biological control; and (e) public education. Perhaps the most important message was that infection knows no national boundaries, and we will pay dearly if we ignore the smoldering of infection anywhere. No single infectious agent is a unique target of concern. During the committee's deliberations, multiple-drug-resistant tuberculosis emerged as a major public health threat in New York City; dare one say this was predictable from the time of its first appearance a few years ago in immuno-compromised people with AIDS? Historically, the influenza of 1918-19 was the most recent previous pandemic to have a major effect on U.S. mortality statistics. Many believe a recurrence of such a lethal reassortment strain, entering a now nave human herd, is all but inevitable. At the moment, HIV is evolving toward greater serodiversity (potentially frustrating proposed vaccines) and occasional azidothymidine-resistance. But its fundamental biological properties, to our good fortune, have not evolved toward easier spread; to the contrary, there are hints of the appearance of strains with longer latent periods, or perhaps associated with very limited pathogenicity. On the other hand, simian virus strains have evolved with aggressive and acute lethality (there is no way to guarantee that the same cannot occur with HIV in the human population), and the potential for genetic recombination with related lentiviruses have scarcely been thought about. A multitude of other zoonoses foster scary outbreaks and importations of Lassa, Marburg, and Ebola viruses. Changing vector distributions, ill-heeded, open the way to a family of encephalitides. Economic and technical advances confer many defenses against infection. Nevertheless, the sheer increase of the global population, its local density in conurbations, and the scale and ease of rapid transoceanic travel are discontinuities in the human condition that aggravate our vulnerability to emergent infections. Some of these are new genetic variants; many others are diseases all too common in their historic habitats and now have new opportunities to spread. Agricultural and residential encroachments on forested lands augment the risk of human contact with arborviruses and borreliae. Other vectors and pests accompany the movement of people and goods. And microbial populations have their own dynamic of evolutionary change connected with their rapid replication, vast numbers, and inherent instabilities--especially on the part of RNA viruses. That the human species has survived so well is a tribute to its evolution of biological immunity, and a modest degree of intelligent self-defense. The human occupation of planet Earth has left us with a conceit that our species is at the very top of the food chain, predatory on all others, rapidly destroying their habitats, and diminishing the genetic diversity of domesticated species. Carnivores that once terrorized human bands have been all but exterminated. Rodents abound, and threaten some food stores; but they can be contained with concrete and warfarin. Our agricultural surpluses are nibbled by insects; but, at their worst, swarms of locusts will be regional, not global, pests. Barring geno-suicide, the human dominion is challenged only by the pathogenic microbes, for whom we remain the prey; they the predator. Our task, then, is to foresee the future of this co-evolutionary relationship, our habitation of Earth in company with large cohorts of microbes. We can discern conflicting trends. Within the infected host, the selective pressures favor aggressive growth correlated with lethality. These invaders are bereft of the evolutionary tradition that tempers the Malthusian fitness of the endogenous somatic cell in favor of the organic individual. But the host rapidly destroyed represents a Pyrrhic victory for the parasite, so efficient transmissibility from host to host is correlated with delayed mortality. Our most effective parasites have entered into our own genome: Every human carries these integrated retroviruses, and is hardly aware of them, even of their possible symbiotic benefits. The future of this relationship is incalculable. At the extreme, the global extermination of our species by disease is hypothetically possible, the most likely in concurrence with the stresses of war and famine. But we need not search far for historic precedents of continental decimation of many plant and animal species, and once every century or so of the human. At stake is a level of sickness and death manyfold higher than that of the highly publicized accidents of industrial pollution. Yet we continue to neglect the most elementary precautions. We have had one wonderful success in the global eradication of smallpox. More problematically, we might accomplish close to the same with polio and perhaps a few other viruses. We certainly cannot aspire to decontaminate the Earth with respect to all potential microbial rivals. These heroic projects deserve our attention, but in keeping with renewed vigilance and a stockpile of knowledge and developmental capacity to cope with an unpredictable variety of emergent pathogens. Our most important need is a well-informed consciousness about these contingencies. Research workers at the front line will add more than any top-down line of command in the generation of new insights. And for operational programs of surveillance and of immunization, the working authorities will do better at designing what is feasible than outside consultants. But none of this will happen without a well-informed, scientific and moral perspective, as well as public understanding and political leadership to allocate the resources needed. Joshua Lederberg is University Professor at Rockefeller University, New York. A version of this essay appeared in ASM News (59[4]:162-3, 1993), published by the American Society for Microbiology. It is used here with the agreement of ASM.

AIDS Pandemic Provokes Alarming Reassessments Of Infectious Disease

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