What's more, it has traditionally been thought that a new disease would show up because people were encroaching on wild areas and contacting animals in their environment, says Stephen Higgs, editor of the journal Vector-Borne and Zoonotic Diseases. And that's still a ripe possibility in many parts of the world. But "with monkeypox -- and to a certain extent, SARS -- we've actually brought the wild animals to us, and into our homes," he points out.

It's difficult to predict which infections will emerge where, says Beatrice Hahn, and "there is no way to keep the lid on the Pandora's box....What you cannot predict is what's going to come out. But what you can predict is that something will."

We agree that it's next to impossible to cast predictions about what diseases will emerge next. But, with some help from those who know best, we're giving it a shot anyway.

Lassa fever
Lassa fever first attracted attention in 1969, when two missionary nurses in Jos, Nigeria contracted a strange fever while caring for patients in the mission hospital.

The Lassa virus. Image: CDC

A hemorrhagic fever similar to Ebola, Lassa now produces periodic epidemics in West Africa. It kills as many as half of all victims. The natural host species is a small rat. Lassa virus spreads easily and can have a long incubation period. The disease kills about 15,000 people a year. A quarter of those who develop symptoms die, and half develop permanent brain damage.

Epidemiologists say that people traveling from endemic areas in Africa could harbor the virus and unknowingly introduce it to new places. So far, Lassa fever has appeared only in spurts, in contrast to viruses (like AIDS) that have "jumped" the species barrier and established a permanent home in people. But the more often Lassa virus infects people, the more opportunities it has to adapt.

Nipah virus
In 1998, a new virus appeared without warning in southern Malaysia. More than 40 percent of people who contracted the disease died, and more than a hundred people died as public health teams slaughtered more than a million pigs to stop the disease's spread. Scientists think the natural hosts of the Nipah virus are large fruit bats that gather in the orchards lining the pig farms of Malaysia's southern peninsula. It would only have taken a few unlucky pigs to spread the infection to people.

Public health experts speculate that the severe El Niño of 1997-98, when the nearby islands of Indonesia were ravaged by drought and forest fires. The fires may have driven Indonesia's bat population north into Malaysia, increasing the availability of Nipah virus to other animals.

A report released this year suggests that the Nipah virus, and the related Hendra virus, may be more widespread in Southeast Asia than anyone expected. Teams of virus hunters are now combing the forests of Southeast Asia to better understand where the viruses linger, and how to best prevent their future spread.

Rift Valley fever
"If it makes it here, that's what I worry about," says Ken Gage. He's talking about Rift valley fever, a powerful, fever-causing viral disease can infect domestic animals (like cattle, goats, and sheep) and people. Outbreaks usually happen during years of very heavy rainfall, when large numbers of mosquitoes transport the disease quickly. It was first reported among livestock in Kenya in the early 1900s, but it now exists in most countries of sub-Saharan Africa. In 2000, an outbreak was reported in Saudi Arabia, and then in Yemen. They were the first cases of Rift Valley fever identified outside Africa.

Unlike many zoonotic diseases, Rift Valley fever can be devastating to domestic animals. The virus survives because many infected animals show only mild symptoms. Others -- in which the virus multiplies wildly -- progress to hemorrhagic fever, encephalitis, and can die. People get it from mosquito bites or from contact with infected livestock. It is untreatable, and so feared by public health experts that it is a priority disease for the Food and Agriculture Organization's Emergency Prevention System. Because mosquitoes are easily carried long distances by wind (and now, by plane, train and car), the virus could spread abroad as West Nile fever has done so successfully.

Map: CDC

Yellow Fever
In September of 2002, a hospital in Dakar, Senegal, admitted a young student bleeding from the nose and mouth, and showing signs of kidney failure. It was yellow fever, a disease caused by a wicked (and very contagious) virus that has been around for at least 400 years. By October, the WHO had reported 15 more cases and warned that an explosive epidemic might be pending. A mass vaccination campaign stopped this from happening, but vaccine supplies are short and not yet widely available.

The yellow fever virus dwells among the mosquitoes and monkeys that live in tropical forest canopies. In many tropical countries, loggers are penetrating deeper into forests, opening a window to the virus to meet with people. The disease is not transmittable from person to person, but epidemiologists warn that the mosquito vectors might introduce the virus into sprawling cities. Once there, an epidemic would be hard to control.

And about that pet hedgehog ...

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