Researchers at the Centers for Disease Control say they do not know why random blood tests found that nearly 3 percent of American adults show signs of previous infection by a family of sometimes fatal viruses that led to a temporary ban on the importation of research monkeys.

The ban was imposed after an infection resembling the fatal Ebola virus turned up in a shipment of dying monkeys imported last fall by Hazleton Research Products in Reston. The virus had never been seen before in a monkey. Although no human illnesses were reported as a result of contact with the animals, blood tests of 550 people who worked with the monkeys found that 42 tested positive for infection with a filovirus, a family of viruses that includes Ebola. In the late 1970s, hundreds of people died in Africa after contracting Ebola.

When CDC tested the blood of 449 randomly selected adults who visited outpatient medical clinics, 12, or 2.7 percent, tested positive for exposure to a filovirus.

"The rate was a bit of a surprise to us, and it deserves more investigation," said Kenneth Herrmann, deputy director of CDC's Division of Viral Diseases.

No one with signs of past infection has become sick, according to Herrmann, who said that the tests may mean that while the virus resembles the deadly Ebola, it is not harmful to people.

The finding also may bolster the theory that the filovirus family includes some that haven't been previously identified.

The federal government recently allowed into the U.S. the first shipment of monkeys since the ban was imposed; it has also licensed a Rockville firm to test animals for Ebola and other filoviruses. The federal government now requires monkeys imported for research to test negative for filovirus before being released from quarantine.