"Indeed there could be different pathotypes [degrees of virulence] of the Marburg virus," said Fred Murphy, a virologist at the University of California in Davis.
But, he added, "when you see the pathology of human infections caused by Ebola or Marburg, the question becomes, How does anyone survive? The infection is devastating, rapid course, with extreme damage to key tissues."
But some scientists suggest that the lethality of the virus may be due to the overreaction of the innate human immune systemour first line of defenserather than the virus itself.
"Any virus worth its salt causes as little damage as possible to its host," said David Sanders, a biologist and Ebola expert at Purdue University in West Lafayette, Indiana. "We run around with viruses all the time. [These are] adapted to us and in equilibrium with our immune system, which doesn't get too aggressive about trying to eliminate them."
Sanders suggests that when a new virus, such as Marburg, enters humans, it may provoke an overreaction of our innate immune system, which in turn causes harm to the body.
"Humans are not a natural host of this virus, so [the virus] wouldn't have evolved to provoke this response," he said. "Instead it may be that the immune system responds inappropriately."
The theory does not explain why most of the victims in the Angolan outbreak have been children. While the innate immune system isas the name impliespresent from an early age, the adaptive immune system develops slowly.
"I would hypothesize that an inappropriate response is responsible for the pathology, while an appropriate response can allow one to survive," Sanders said. "Stopping viral multiplication can reduce the triggering of the harmful reaction."
There is also the possibility of a difference in the susceptibility of individuals due to general health status and nutrition, scientists say.
Angola is one of the poorest countries in the world. Its infrastructure was wrecked by a two-decade civil war that ended in 2002. Sanitation facilities are inadequate or non-existent, and hospitals are understaffed and poorly equipped.
The prospect of the virus gaining a foothold in Luanda, almost four million people live in the Angolan capital, is ominous.
"The first thing that comes to mind is the international airport," said Daigle, the CDC spokesman. "We saw with SARS how fast it was able to spread to Canada and other countries once people started getting on planes."
Containing an outbreak is more difficult in a densely populated area where people are crowded together. The virus has the potential to rapidly spread to other people, especially health-care staff and family members who care for patients.
At Americo Boa Vida, Angola's largest hospital on the outskirts of Luanda, a special isolation ward has been created to treat incoming cases from around the country. Volunteer workers have been outfitted with special suits to work as cleaners and washers.
"The practices needed to contain the virus in the hospital setting are rather simple and can be quickly introduced by international teams," Murphy, the UC Davis virologist, said.
A team of CDC scientists will arrive in Angola this week to assist WHO officials already there with outbreak investigation, infection control, and laboratory diagnosis.
"Our guys are very anxious to get on the ground and start doing some testing and contact-tracing to find out if this is a case of an unusually high fatality rate, or if we're just not able to track all those who are sick," Daigle said.
The outbreak also presents a valuable opportunity for scientists to learn more about the virus. Scientists don't know the environmental reservoir for either Marburg or Ebola. (A reservoir is where a virus hides between outbreaks.) But most experts believe the two viruses share the same host.
"We got some evidence, but not conclusive, that bats may have been the source of the Marburg outbreak in the mining village of Durba in the Congo," said Bob Swanepoel, who heads the Special Pathogens Unit at the National Institute for Communicable Diseases in Sandringham, South Africa.
"Genetic evidence of the virus was found in cave-dwelling bats in the mine where primary human cases arose, but we were not able to isolate live viruses from bats, and the outbreak stopped when the mine flooded," he said.
Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES