The Ebola and Marburg viruses have been known to modern science for less than four decades. In that time, they have killed about 2,000 people in total. Yet the merest mention of an outbreak evokes fear and dread, due to the deadly nature and rapid spread of these similar diseases.
The Marburg and Ebola viruses were identified in 1967 and 1976 respectively. Both are filoviruses - so named because of their filament-like structure. They are symptomatically almost identical, causing fever, headaches, impaired liver and kidney function, and ultimately, both internal and external bleeding.
The World Health Organization describes them as among the most virulent pathogens known to infect humans, ultimately killing 50 to 90 percent of those infected.
Ebola is known to have four major strains. Three of them, which occur in Africa, cause illness in humans. The fourth, which occurs in the western Pacific, infects humans, but causes no symptoms.
Marburg, while first identified in Europe, only occurs naturally in Africa. The European outbreak was caused by contact with infected green monkeys, imported from Africa for research.
Both viruses are transmitted between humans, or between humans and infected primates by close contact. Scientists believe that these infections take place primarily through tissue and blood contact, and that respiratory transmission, if it does occur, is rare.
But science has not identified the natural host, or reservoir, for either of the viruses, nor how they are transmitted from the reservoir to primates and humans.
The World Health Organization, or WHO, says investigators have tested hundreds of animals, insects and plants in search a reservoir, but have not found conclusive evidence of an animal or environmental source.
The WHO says some bats infected with the Ebola virus do not die from it, raising speculation that they could play a role in maintaining the virus in the tropical forests where Ebola appears to reside.
Dr. Robert Swanepoel, consultant to South Africa's National Institute for Communicable Diseases, says researchers there have been researching the bat theory, both for Ebola and for Marburg. He explains the research is in its early stages and remains inconclusive.
Dr. Swanepoel describes what researchers discovered while investigating an outbreak of Marburg in the Democratic Republic of Congo that killed 128 people. "And, we, ourselves, found evidence, nucleic acid -- we haven't published this yet - but we did find nucleic acid in, in other words genetic material of the Marburg virus in bats in a gold mine in northeastern DRC, where there was an outbreak of Marburg from October 1998 until September 2000, in this mine," Explained the doctor.
He said researchers tracking the path of infection were able to link people who were infected with someone who had worked in the mine. "Every time, one (researchers) controlled the chain of transmission within a family, the father would get sick in the mines, and then the wife, and so forth. You would control that, and then, there would be another outbreak in a week or two," explained Dr. Swanepoel. "We found these were different strains of the virus, if you could call them strains. We found they were all picking it up in a gold mine. The mine flooded and the disease disappeared. So, that brought us nearer to the idea of bats."
Outbreaks of Ebola seem to have strong connections to forested areas, with those infected first in an outbreak having spent time in a forest, or having eaten a monkey found dead in a forest. Dr. Swanepoel says that, once again, the local research suggests bats could play some sort of role.
"Way back in 1996, we, ourselves here, infected a whole lot of different animals, which we believed were potential hosts, and even plants and so on, with Ebola," he said, "and we found that ... fruit and insect bats were able to replicate high levels of virus in their circulation, and not succumb to any disease that we could determine."
Dr Swanepoel emphasizes the research is inconclusive, and that the South African findings are part of a much larger and currently incomplete puzzle. He says further research may rule bats out.
Researching these viruses has proven difficult for scientists, not least because outbreaks usually occur in remote, hard-to-access areas. Dr. Swanepoel says conducting research during an outbreak, when communities are frightened and suspicious, can be difficult.
"On the other hand, if you wait until it is all over, then you may have missed it," he notes. "The source might be seasonal - for argument's sake, it might be during the breeding season of the bats - then once you go in after an outbreak, it's all over."
And, says Dr. Swanepoel, researchers encounter other difficulties, such as securing funding and getting permission from authorities in countries where an outbreak has occurred.
Until a vaccine is developed, or a cure is found, the best hope for preventing rapid spread of either Ebola or Marburg is early detection and containment to prevent transmission of the disease.