Ebola virus, formally called Zaire ebolavirus, is a rare virus that infects humans and nonhuman animals such as pigs and other primates. It is one among several viruses within the genus Ebolavirus, only four of which are known to infect humans: Ebola, Sudan, Taï Forest, and Bundibugyo.

Some viruses within the genus Ebolavirus do not cause symptoms in humans, such as Reston ebolavirus, the variant at the center of the book and TV series The Hot Zone (though it does cause disease in pigs and nonhuman primates). A variant called Bombali virus was recently found in bats, but at present it's unclear whether it infects other animals. Zooming out, Ebolavirus sits within a group called the filoviruses, which includes similar