Some people exposed to the Ebola virus quickly sicken and die. Others become gravely ill but recover, while still others only react mildly or are thought to be resistant to the virus. Now researchers working with mice have found that these laboratory animals, too, can have a range of responses to Ebola, and that in mice, the responses are determined by differences in genes.

This is the first time scientists have been able to breed mice that developed Ebola infections resembling those in humans complete with some puzzling features seen in people.

About two-thirds of people who die from Ebola never develop the terrifying hemorrhages that appear in others a day or two before death, in which eyes turn fiery red, gums bleed, red dots emerge on the skin as blood seeps out of capillaries, and blood appears in vomit and diarrhea. Many mice, too, die of Ebola without hemorrhages.

The mouse studies indicate the animals that hemorrhage and — by implication, humans— die because their immune systems overreact to the virus. The result is an inflammatory response that makes cells leak fluids and white blood cells, and makes tissues and organs deteriorate. Many die at that point. In those mice — or humans — that survive long enough, the researchers propose, blood eventually starts to seep out of vessels.