A mysterious lethal virus has broken out in Zaire, killing at least 56 people and prompting government officials to place parts of the country under quarantine, health officials said yesterday.

The most likely cause is Ebola -- a highly contagious disease that can be fatal in 90 percent of cases, causing its victims to bleed through their pores and bodily orifices -- or a related hemorrhagic fever virus, officials with the Centers for Disease Control and Prevention said yesterday.

As of Sunday, Zaire told the World Health Organization, it has 172 cases of the ailment in Kikwit, a city of 600,000 which is about 250 miles from the Zairean capital of Kinshasa. Among the ailing are 24 health-care workers, including four Italian nurse-nuns. Reports from Kinshasa to WHO suggest that two of the nuns may have died.

The CDC's Special Pathogens Laboratory in Atlanta, the world's highest security medical facility, received blood and tissue samples drawn from victims of the outbreak yesterday at 10 a.m. It hopes to provide a definitive identification this morning.

"If it is Ebola, this [disease] is the big one -- this is what we're always thinking about when we talk about serious, dangerous disease threats," said Dr. James Le Duc, head of the World Health Organization's special virus group.

Ebola has received a tremendous amount of popular attention lately, having been the focus of two books, "The Hot Zone" and "The Coming Plague," a motion picture, "Outbreak," and an NBC-TV movie special that aired Monday night, "Virus".

Ebola outbreaks have only been known to occur three previous times, all in Africa: in Yambuku, Zaire, in 1976, where it killed 274 people, and in Nzara, Sudan, in 1976 and 1979. In those epidemics, mortality rates among the infected villagers and medical personnel ranged from 70 to 95 percent.

Information on the current epidemic is scarce. According to Le Duc, his organization didn't learn of the outbreak until Sunday. At that time, Zairean officials told him that the epidemic began on April 10.

Word of the outbreak was delayed by civil war conditions in Zaire, and because the virus samples were sent first to Belgium -- even though Belgium no longer funds a Biosafety Level 4 laboratory, designed to contain the most dangerous viruses. The samples were then sent on to Atlanta.

The Zairean government has requested international assistance, and the WHO's Dr. David Heymann, who was a member of the scientific team that investigated the 1976 Yambuku epidemic, arrived there last night. The CDC is also assembling a crew in Atlanta that will be dispatched "within two to three days," said Dr. Ruth Berkelman of the emerging diseases division.

The scramble is now on to find equipment that will allow the scientists to work safely in the field with the super-lethal virus.

The Ebola virus belongs to a class of organisms called filoviruses, which destroy the linings of capillaries and blood vessels, prompting fluids to drain out of the circulatory system. The viruses' course is painful, and victims typically become deranged and manic before dying of shock.

Whether the virus can be spread in the air is a point of great controversy for Ebola. Though some scientists believe that there is evidence that airborne transmission has occurred, at least between monkeys infected with a strain called Ebola Reston, the WHO considers blood contact to be the primary mode of spread.