Some of the viruses are notorious, such as Ebola and HIV, others have less familiar names: Marburg and Lassa fever. they have all emerged in recent decades from sub-Saharan Africa, perplexing scientists and, in the case of HIV, killing millions.

Africa is recognized as an ideal incubator for new pathogens: It has rapidly growing human populations and high biodiversity, along with widespread poverty, poor medical care and, in many areas, armed conflict that forces civilians to flee far from their homes.

``For every virus that we know about, there are hundreds that we don't know anything about,'' said Dr Dan Bausch, a professor at Tulane School of Public Health and Tropical Medicine who studies Marburg, Ebola and other emerging diseases in Africa.

``Most of them, we probably don't even know that they're out there.''

Scientists are especially baffled by Marburg. Since 1967, the Marburg virus and its equally lethal cousin, Ebola, have killed more than 1,600 people.

The latest Marburg outbreak has killed at least 277 people in Angola, hundreds of kilometers from where it last emerged four years ago, in the Democratic Republic of Congo.

``To be honest with you, I have no indication what the source is,'' said Dr Pierre Formenty, the World Health Organization's senior Marburg expert, at the epicenter of the outbreak, in Uige, Angola. ``That was not our first priority. Now we are working on it.''

In many cases, it takes years of research to identify the viral reservoir, the animal or plant species that is host for the pathogen, before its transmission to humans.

Researchers believe one key cause of emerging disease is the rapid growth of human population in tropical regions that have an abundance of animal species. When people encroach on once-remote animal habitats, opportunities for human-to-animal contact grow, as do the opportunities for a virus to jump from an animal host to people.

``Areas rich in biodiversity are rich in biodiversity of microbes,'' said epidemiologist Stephen Morse, a professor at Columbia University School of Public Health who is trying to develop better early-warning systems for diseases such as Ebola and avian influenza. With so many tiny organisms around, he said, it stands to reason some will infect humans with lethal results.

``People probably used to get it from time to time,'' he said. ``But in the old days, with lower density, it was hard to spread.''

Jonathan Patz, an expert on the link between disease and ecology at the University of Wisconsin and Johns Hopkins Bloomberg School of Public Health, said almost three-quarters of newly emerging diseases come from wildlife.

Hong Kong and the rest of the world are not immune from the latest virus, in part because an infected person can travel around the globe by aircraft.

``When you've got something like this in Angola, it seems dreadful,'' said Bob Swanepoel, retired head of the special pathogens unit at South Africa's National Institute for Communicable Diseases. ``The same sort of things happen in developed countries; it's just that there is better medical attention. We don't have exclusive rights on these things in Africa.''

Africa is infamous for dangerous new viruses in part because of the notoriety of Ebola, first identified in 1976.

Ebola and Marburg begin with sudden fever and aches, followed by vomiting and diarrhea and, within days, uncontrollable bleeding. Both are filoviruses, named for their threadlike appearance under a microscope.

Person-to-person transmission is caused by close contact with contaminated blood and other body fluids. People have been known to get Ebola by handling infected chimpanzees, gorillas or antelopes.

Mortality rates vary. The three Ebola strains that affect humans kill 50 percent or more of their victims. Marburg kills more than one-quarter of its victims; early figures from Angola are much higher, with a death rate of about 90 percent.

Researchers have long known that people can contract the viruses from monkeys. But how do monkeys contract them? Marburg and Ebola kill primates, which means those animals are not the viral reservoir.

The viruses cannot survive on their own; they exist inside the cells of other creatures that have acquired at least partial immunity.

Bats are one leading possibility. In the late 1990s, 128 gold miners in the Democratic Republic of Congo died of Marburg. Tests on bats found genetic evidence of the virus, but subsequent tests have been inconclusive. But when the mine flooded, depriving the bats of their habitat, the outbreak ended.

Three of the five other Marburg outbreaks also pointed to bats, said Swanepoel, including two in which the victims visited the same cave in Kenya seven years apart. He speculates the victims came into contact with bat droppings and were infected after rubbing their mouths or eyes.

Formenty said there is no indication bats are to blame for the outbreak this year in Angola.

Patz and others argue that environmental degradation plays a key role in the emergence of new diseases. People all over West and Central Africa are moving into previously uninhabited ecosystems.

Loggers build roads into the jungle; hunters and settlers follow, using the roads to reach new regions. They come into contact with microbes for which they may have little or no immunity.

Environmental degradation can also spread disease by increasing the prevalence of certain bugs. Patz has found that areas where dams are built often see increased incidence of mosquito-borne diseases, such as malaria and Rift Valley fever.

Dams raise the water table and increase the amount of standing water, creating ideal conditions for mosquitoes. As the mosquito population rises, so do rates of the diseases they bear.

Such inadvertent shifts are widespread, Patz said: ``We are disturbing natural systems more rapidly and in a more widespread way that at any time in history.''

Many places have high biodiversity, but Africa has more primates than anywhere else, particularly species that are closest to humans, such as chimpanzees and gorillas. Infectious viruses have a relatively easy time hopping between species with similar genes.

``The more closely related to species you are, the more likely a disease will jump,'' said epidemiologist Nathan Wolfe, a professor at the Bloomberg School of Public Health. ``What's key about that is almost all of our closest relatives are in Africa.''

This mode of transmission probably has occurred. The most prominent example is the human immunodeficiency virus, which can cause AIDS. It is believed to have derived from a virus that infected chimpanzees.

Most experts believe the disease made its way into human beings over an untold number of years through hunters who butchered chimpanzees and ate infected meat.

At some point, the virus seems to have mutated inside humans, allowing it to survive and spread among people.

Ebola might have arrived by a similar route, said Wolfe.

Over the past few decades, several Ebola outbreaks seem to have started after people ate dead primates they had found in the jungle.

Emerging diseases have spread from Asia, among them the Nipah virus, which killed more than 100 people in Malaysia in 1999; severe acute respiratory syndrome, or SARS, which killed more than 700 people in 2003; and avian flu, which has killed about 50 people.

More than 100 million birds have died of the disease or been killed as a precaution.

South America, with conditions similar to those of Africa, has produced diseases, too, and experts expect that to continue.

``The changes going on in the Amazon are just as likely to produce significant disease as anywhere,'' said Peter Daszak, a disease ecologist who directs the Consortium for Conservation Medicine in Palisades, New York.

Many more new diseases appear around the world than anyone realizes, he said.

In most underdeveloped countries, there is little disease surveillance. As a result, many bugs might exist for long periods without being detected.

Daszak points to the Nipah virus. After it first infected humans in Malaysia, officials there watched carefully for its reappearance.

Last year, researchers in Bangladesh, several hundred kilometers to the northwest, realized that the virus had been infecting people in that country for four years.

No one noticed because no one had really looked.

``These viruses emerge in different countries,'' said Daszak. ``Sometimes they're recognized, and sometimes they're not.''