Don't worry, it can't hurt you—yet.

Scientists have identified Reston ebolavirus—a member of the deadly Ebola group of hemorrhagic viruses—in domestic swine from the Philippines.

The virus, which looks like a piece of yarn with a slight bend, is the only Ebola pathogen not known to cause disease in humans. Even so, the U.S. Centers for Disease Control and Prevention in Atlanta considers it a biosafety level 4 pathogen, reserved for the most dangerous and exotic diseases.

Ebola and the closely related Marburg viruses are highly contagious, causing vomiting, diarrhea and bleeding with death rates as high as 90 percent. These viruses, originally from Africa, are thought to be caught from close contact with monkeys and apes, their primary hosts, although they have also been isolated from bats that show no symptoms.

Indeed, Reston ebolavirus was first identified in 1989 in crab-eating macaque monkeys that were shipped from the Philippines for research in Reston, Va. Human caretakers developed an immune response to the virus, but they never came down with any symptoms.

The latest outbreak of the Ebola family was discovered in July 2008 as the Philippine Department of Agriculture was investigating "blue ear disease" in pigs, a respiratory condition that causes their ears to turn blue from lack of oxygen. Investigators sent tissue and blood samples to Michael McIntosh at the U.S. Department of Agriculture's Foreign Animal Disease Diagnostic Laboratory at the Plum Island Animal Disease Center in Greenport, N.Y.

McIntosh says he was surprised to find that the tissue samples also contained the Reston strain, which had not been previously identified in swine. His team also confirmed pig-to-human Ebola transmission by identifying six pig handlers, whose blood tested positive for antibodies to the virus, although they showed no symptoms. Manila had announced preliminary findings in January, and McIntosh's study is published in this week's Science.

McIntosh says there are still a lot of unknowns, including how the virus was transmitted to the pigs and whether they show any symptoms independent of blue ear disease.  He worries that the virus's passage through pigs could allow it to mutate into something more harmful.  The research also raises the possibility that pigs could be infected with lethal Ebola strains. "What is the level of risk? We really don't know," he says, "The fact that it shows up in domestic pigs raises that risk."