11.   Viruses like Ebola are usually carried by animals, which generally remain healthy.

12.   Animals can carry the highly contagious virus for up to two weeks without showing symptoms, and officials had hoped the number of new cases would being falling.

13.   Many animals can carry the disease without catching it themselves.

14.   The bone showed evidence that an animal could have carried it away from the rest of the remains, they said.

15.   The next step is to look at what could happen if the animals being released carry an infection that could contaminate the animals already in the wild.

16.   The new animal carries the same traits.

17.   Use an encyclopedia to find out which animals most often carry the deadly virus.

18.   But Weissmann said that uncertainty remained about whether animals could carry the agent without becoming ill.