Infected Planet

When Michael Crichton's first novel, "The Andromeda Strain," was published in 1969, it was scary but also strangely reassuring. If some new disease were to threaten humanity with a deadly pandemic, it seemed, the microbe responsible would come from another planet. The march of medical progress appeared to have terrestrial germs on the run.

Twenty-five years later, when Laurie Garrett published her nonfiction bestseller, "The Coming Plague," people were waking up to the fact that our own abused planet is perfectly capable of spawning a steady stream of new diseases without any help from alien worlds.

Today, old familiar scourges like tuberculosis, malaria, measles, and diarrhea -- and a newer one, AIDS -- are the world's biggest killers, but they've been joined by a host of newcomers. Indeed, one could get the impression that each year brings a new disease. That's because it does.

Mark Woolhouse, chair of Infectious Disease Epidemiology at the University of Edinburgh, has counted 38 new pathogens (disease-causing biological agents) that have moved into the human population from other animal species in just the past 25 years. In a presentation at the annual meeting of the American Association for the Advancement of Science last month, Woolhouse noted that we're under assault not only from those novel species, but also from new genetic variants of pathogens that have been with us for a long time.

A recent tally identified 1,415 disease-causing microbes in humans, including bacteria, viruses, fungi and parasitic worms. We share fully 61 percent of those pathogens with other animal species. Of the total, 175 cause "emerging diseases" -- ones not known until recently in humans. Of those, 75 percent came out of other animals to invade Homo sapiens.

The impact of species-jumping pathogens varies. Hendra virus moved from fruit bats to horses in 1994 and is known to have killed a total of only three people. Since the 1970s, the Ebola virus has incited some horrifying outbreaks that, so far, have failed to blow up into epidemics. Influenza viruses usually cause a lower mortality rate but hit far more people; currently, an H5N1 "bird flu" strain threatens to break that pattern by staging an encore of the 1918-19 killer flu pandemic that killed 50 million to 100 million people. HIV/AIDS is both chronically widespread and deadly, now accounting for almost a fourth of infectious disease deaths.

But have "emerging" species-jumping diseases actually been with us for millenia, identified only when medical research achieves sufficient precision in detecting and identifying microbes? Durland Fish, professor at the Yale School of Public Health, says that better research is part of it, but there still appears to be a faster rate of disease appearance these days. He told me, "Dr. Woolhouse makes an interesting point: that 'emerging disease' is a new concept but a very old process. Humans have always acquired new diseases." We're being hit more frequently today than in previous eras, he says, partly because "transportation, trade, human population growth, and environmental change are going on at unprecedented rates."

They don't show up uninvited

Scientists have seen associations between human activites, which have burgeoned in the past quarter century, and diseases that gained prominence during those same years. Some examples:

The chances of the potentially catastrophic flu virus H5N1, and others like it, emerging from interaction between wild birds, domestic animals and people may have been enhanced by loss of natural wetlands in southern China. That has led infected migratory birds to alight more often on farms and other populated areas. There, they come into contact with denser populations of chickens, ducks and pigs destined to satisfy an increasing rate of animal-protein consumption per person.