Scientists have identified a total of five species of ebolavirus, four native to Africa and one to the Philippines. They are all zoonoses, meaning animal infections transmissible to humans. They reside quietly in some species of wildlife, this or that forest creature, from which they spill over occasionally to cause mayhem and death in people. Ebola virus can only pass from person to person by direct contact with bodily fluids, and therefore an outbreak is stoppable by simple isolation and “barrier nursing,” or the careful handling of patients and corpses, once enough medical gloves, gowns, goggles, rubber boots, body bags and knowledge have reached the scene.

Although the outbreak is eventually halted, the virus isn’t gone. It hides in the forest within some hospitable animal, its reservoir host. The identity of the reservoir host (or hosts) for Ebola virus is unknown, but three species of fruit bat are suspected. One of those species, the hammer-headed fruit bat, lives in forests from the Congo basin as far west as southeastern Guinea and is sizable enough to be attractive as human food.

In Guinea, the index patient (the first case) has not even been identified, though the preponderance of cases in Guéckédou, a city near the southeastern border, suggests that the outbreak may have begun down there. Without knowing the index patient, investigators can’t know how the fateful spillover happened. Did someone eat a bat? Did someone scavenge another dead chimp, one that had shared fruit with a bat?

That sort of research will have to come later, I was told recently by Dr. Stuart Nichol, of the Centers for Disease Control and Prevention. Dr. Nichol, himself a veteran of many outbreak responses, is chief of the C.D.C.’s Viral Special Pathogens Branch, a portfolio containing all the world’s nastiest viruses. Some of his people are presently in Guinea. For now, Dr. Nichol said, the focus is on human health: identifying cases, getting them isolated, giving them supportive care (there are no real treatments for Ebola virus disease), tracing contacts, breaking the chains of transmission.

The original Zaire species of ebolavirus was the first recognized back in 1976, when it emerged at a place called Yambuku in the northern boondocks of what then was Zaire, now the Democratic Republic of Congo. Of the 318 people infected, 280 (88 percent) died. A later outbreak in northeastern Gabon, in 1996, began with a chimpanzee carcass, scavenged from the forest by a group of boys on a hunting foray and devoured communally back at their village. (Chimps cannot be the reservoir host of Ebola because they too die from it, as this one evidently had.) Thirty-one human cases, 21 deaths. The Zaire virus has also struck three times in the Republic of Congo just across the river, killing dozens of people each time.