Hemorrhagic diseases like the one that apparently has struck the city of Kikwit in Zaire have been around for many years, and they remain a horrible way to die, according to a Johns Hopkins University expert on the viruses.
And more of them seem to be turning up as humans move more easily in and out of once-remote regions where the viruses had lived isolated in animal hosts.
"What you have is the potential to expose lots of people, over areas very far removed from the initial outbreak, so you have a much larger pool of people that could be at risk," said Dr. Gregory Gurri Glass.
Dr. Gurri Glass, 42, is an associate professor in the department of molecular microbiology and immunology and Hopkins' School of Hygiene and Public Health.
He is an expert on the hanta hemorrhagic virus that struck in 1993 in parts of Arizona, New Mexico and Utah.
Hemorrhagic viruses have little in common except that, in humans, they produce symptoms that include hemorrhaging, or bleeding. The hanta, yellow fever and dengue fever viruses are among the more familiar varieties.
More recently identified varieties include the Ebola and Marburg viruses that killed several hundred people in Africa in the 1970s and 1980s. Others include the Lassa, Crimean Congo and Bunya viruses. All are named for the places where they first were identified.
"They seem to affect the cells that line the insides of our capillaries and blood vessels," said Dr. Gurri Glass. "They're sort of the waterproofing that keeps the fluid from leaking out."
Patients develop internal bleeding, which can produce bruising under the skin, and bleeding in the gastrointestinal tract that leads to bloody vomiting and diarrhea.
Tissues lining the ears, eyes and nose may also begin to bleed.
"You lose blood pressure and blood volume, and consequently you don't get good oxygenation of organ systems. . . . Lots of organ systems are failing," he said.
Death comes in a week to 10 days. Ebola can kill 90 percent of its victims. Bunya kills 15 percent to 20 percent.
Treatment includes attempts to replace the lost fluids, but that may send fluids into the lungs. "It's a very difficult thing for physicians to treat," he said.
Vaccines are difficult to develop, Dr. Gurri Glass said, so the best strategy for fighting an outbreak is to enforce "well understood but rarely applied public health measures," including isolation of the sick, their families and the health workers who care for them.