That’s why experts say it’s critical to understand underlying causes. “Any emerging disease in the last 30 or 40 years has come about as a result of encroachment into wild lands and changes in demography,” says Peter Daszak, a disease ecologist and the president of EcoHealth.

Emerging infectious diseases are either new types of pathogens or old ones that have mutated to become novel, as the flu does every year. AIDS, for example, crossed into humans from chimpanzees in the 1920s when bush-meat hunters in Africa killed and butchered them.

Diseases have always come out of the woods and wildlife and found their way into human populations — the plague and malaria are two examples. But emerging diseases have quadrupled in the last half-century, experts say, largely because of increasing human encroachment into habitat, especially in disease “hot spots” around the globe, mostly in tropical regions. And with modern air travel and a robust market in wildlife trafficking, the potential for a serious outbreak in large population centers is enormous.

The key to forecasting and preventing the next pandemic, experts say, is understanding what they call the “protective effects” of nature intact. In the Amazon, for example, one study showed an increase in deforestation by some 4 percent increased the incidence of malaria by nearly 50 percent, because mosquitoes, which transmit the disease, thrive in the right mix of sunlight and water in recently deforested areas. Developing the forest in the wrong way can be like opening Pandora’s box. These are the kinds of connections the new teams are unraveling.

Public health experts have begun to factor ecology into their models. Australia, for example, has just announced a multimillion-dollar effort to understand the ecology of the Hendra virus and bats.

IT’S not just the invasion of intact tropical landscapes that can cause disease. The West Nile virus came to the United States from Africa but spread here because one of its favored hosts is the American robin, which thrives in a world of lawns and agricultural fields. And mosquitoes, which spread the disease, find robins especially appealing. “The virus has had an important impact on human health in the United States because it took advantage of species that do well around people,” says Marm Kilpatrick, a biologist at the University of California, Santa Cruz. The pivotal role of the robin in West Nile has earned it the title “super spreader.”

And Lyme disease, the East Coast scourge, is very much a product of human changes to the environment: the reduction and fragmentation of large contiguous forests. Development chased off predators — wolves, foxes, owls and hawks. That has resulted in a fivefold increase in white-footed mice, which are great “reservoirs” for the Lyme bacteria, probably because they have poor immune systems. And they are terrible groomers. When possums or gray squirrels groom, they remove 90 percent of the larval ticks that spread the disease, while mice kill just half. “So mice are producing huge numbers of infected nymphs,” says the Lyme disease researcher Richard Ostfeld.