JOHANNESBURG, South Africa - Some of the viruses are already notorious, such as Ebola and HIV. Others have less familiar names: Marburg and Lassa fever. But they share one feature: All have emerged in recent decades from sub-Saharan Africa, perplexing scientists and, in the case of HIV, killing millions.
Africa is now 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 countries, 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 the 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 remain 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 miles 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, speaking from 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.
A key factor
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 that are rich in biodiversity are rich in biodiversity of microbes," said epidemiologist Stephen Morse, a professor at the 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 says, it stands to reason that some will infect humans with lethal results.
"People probably used to get it from time to time," Morse said. "But in the old days, with lower density, it was hard to spread."
"Almost three-quarters of newly emerging diseases come from wildlife," said Jonathan Patz, an expert on the link between disease and ecology who divides his time between the University of Wisconsin and the Johns Hopkins Bloomberg School of Public Health.
The United States 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 plane.
"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. "In fact, 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 has become 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 a 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. However, when the mine flooded, depriving the bats of their underground habitat, the outbreak ended.
Three of the five other Marburg outbreaks also pointed to bats, Swanepoel said, including two in which the victims visited the same cave in Kenya seven years apart. He speculates that the victims came into contact with bat droppings and were infected after rubbing their mouths or eyes.