In the absence of news reports, bird flu might seem to have flown the coop. Unfortunately, it never actually went away—and is now worse than ever. "There are more flu infections in more countries than ever before," said veterinarian William Karesh, head of the Field Veterinary Program of the Wildlife Conservation Society (WCS), during a WSC conference in New York City last week.

According to the World Health Organization (WHO), 381 people worldwide have been infected—240 of them fatally—with the H5N1 strain of bird flu since 2003. South Korea has suffered through at least eleven outbreaks in poultry this month alone, leading to the culling of 1.9 million chickens and ducks to try to keep a lid on the disease. In Egypt, 50 people, including a two-year-old boy most recently, have been stricken since 2006**; 22 of them have died.

A recent study shows that so-called zoonoses—infectious diseases such as H5N1 bird flu, West Nile and Ebola that strike both animals and people—now account for as much as 58 percent of human pathogens and the number is growing fast. In just the past five years, WHO has identified more than 1,000 epidemics stemming from such pathogens.

These diseases are hard to stop because they are not only spread by wild animals, mosquitoes and the like but, even more commonly, by billions of livestock animals, such as chickens, ducks and geese raised for food in vast factory farms. Zoonotic outbreaks are triggered by a range of factors, including man-made changes to natural habitats that bring humans into contact with wildlife as well as airplanes and other forms of transport that allow "speedy, long-range dissemination of any disease agent," says veterinarian Arnon Shimshoni of The Hebrew University of Jerusalem.

And humans are not the only ones hurt. In their haste to prevent an outbreak, jittery authorities and farmers are sometimes too quick to kill some noninfectious critters, Karesh says. "Wildlife can be killed by a disease, but, more frequently," he notes, "they are killed by humans afraid of getting a disease or afraid of their animals getting a disease."

Even the best-intentioned plans can backfire, such as a conservation scheme to link Krueger National Park in South Africa with similar parks in Zimbabwe and Mozambique. Animals carrying diseases could spread them far and wide. For instance, South African wildlife infected with bovine tuberculosis, could spread it to animals and people in the north as they move more freely across the landscape. Simultaneously, tstse flies from the north might spread trypanosomiasis* south, endangering the recently recovered largest population of white rhinoceroses in the world.

And rabies, a problem in Mozambique, might spread to infect animals in Krueger. "In an area that is ecologically naive to [a] virus, it could wreak havoc," warns veterinarian Steven Osofsky of the WCS Field Veterinary Program.

Perhaps the most telling example of the inextricable link between human and wildlife health is that of our closest kin, the great apes. In the forests of the Congo River Basin—the world's second largest intact tropical forest—chimpanzees and gorillas suffer from the same Ebola virus as humans. This hemorrhagic fever can kill as much as 95 percent of the apes it infects; it has claimed at least 5,000 western lowland gorillas in just the past two years. "The threat of Ebola alone caused the [International Union for Conservation of Nature and Natural Resources] to upgrade western lowland gorillas to critically endangered," veterinarian Ken Cameron of WCS's Field Veterinary Program in central Africa, reported at the WCS's State of the Wild conference. "We could lose 80 percent of the species in the next 10 years."

As a result, the WCS has joined forces with the U.S. National Institutes of Health, Boston University and other research institutions in an effort to adapt a vaccine against Ebola for gorillas. At least one candidate has proved effective in protecting macaques from one of the three strains of the Ebola virus but it remains unclear whether it will work in people or gorillas.

But even if a successful vaccine is developed, it will be no easy task to inoculate notoriously elusive gorillas. "We are seeing less than one tenth of one percent of the total population," Cameron said. Darting them with a vaccine then seems unlikely.

An alternative might be to inject fruit and other food that apes like to munch. But Cameron notes that it would be hard to control doses that way—and that other animals might end up gobbling the medicine. "We hope that vaccines can help, but it's too early to tell," Cameron said. "We do feel we need to try." For the gorillas—as well as for us.

"There is an artificial distinction between the health of wildlife, livestock and people," Karesh said. Sickness in one of these groups, can mean sickness in all.

*Correction (4/24/08): This article originally identified the disease threatening rhinoceroses as rabies.

** Correction (4/25/08): This article originally ascribed the Egyptian statistics to this year instead of since 2006.