Avoiding caves is clear-cut advice, but other routes of infection may be harder to block: People in many parts of the world eat bats, and may be infected while catching or preparing them for cooking. Hunters and cooks may not be able to tell one bat species from another.

The researchers said the findings did not mean that bats should be exterminated. They protect humans and crops by eating insects and pollinating fruit trees. Disrupting complex ecosystems by slaughtering bats could even make disease outbreaks worse.

Zaire ebolavirus is the cause of the current epidemic in the Democratic Republic of Congo, with more than 700 cases and more than 400 deaths. The outbreak, which has spun out of control in a war-torn region, is the second largest ever. The largest, caused by the same Ebola species, occurred in West Africa from 2013 to 2016, infecting nearly 30,000 people and killing 11,000.

The West African epidemic, in Guinea, Liberia and Sierra Leone, is thought to have begun with a small boy in Guinea handling an infected bat, but the origin is not known for sure.

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A Liberian team in full protective gear — Tyvek suits, gloves, masks, goggles, hoods, boots — trapped and released 5,000 bats from about 10 species. They took samples of blood, urine and feces, and oral swabs.

Dr. Anthony’s lab at Columbia found genetic material from the virus in a mouth swab taken from just one bat, captured in Liberia’s northeastern Nimba District. That animal was a greater long-fingered bat, from the species Miniopterus inflatus, a furry beast the size of a small mouse, weighing half an ounce, with a 12-inch wingspan. It eats insects.