It causes fever, vomiting and diarrhea and can cause internal and external bleeding. The virus can be found in all the bodily fluids so health care workers must take extreme care, CDC Director Dr. Julie Gerberding said.

"People are frightened and people are embarrassed to be associated with this but I think the important message is we need to identify those who are sick, get them into care and teach health care workers how to protect themselves from blood and body fluid exposure," Gerberding told CNN television.

There is no specific treatment for Marburg but putting patients on fluids and watching them in the hospital can help them survive.

The CDC chief said anyone who travels to the area and who is in contact with a Marburg victim needs to watch for symptoms for 10 days. But she said she did not believe the virus threatened to spread to the rest of the world.

"We're optimistic about this one," she said.

Gerberding said the CDC had sent eight staffers to Angola to join a World Health Organization team that is helping fight the virus, find and treat patients and, eventually, track down its source.

Scientists believe that Marburg, like Ebola virus, comes from an animal, possibly a monkey or other primate. Studies suggest that people become infected when they hunt and butcher monkeys, apes and other animals for the bush meat trade.

"There is a suspicion that there is some kind of primate source," Gerberding said.

At least 203 people have died from the Marburg virus in this latest outbreak out of a total of 221 known cases. Gerberding said this high mortality rate was unusual, as the virus killed about 25 percent of victims in previous outbreaks.

She said she suspected it was because people were not seeking medical care until they were very ill.