There are plenty of diseases that we contract from animals, but scientists were convinced that hepatitis A wasn't one of them. A look at other diseases shows us that we're wrong — and that bats played a part in passing on the disease to humans.
The virus for hepatitis A does show up in non-humans occasionally, but only in very rare cases and only in primates. A group of researchers headed by scientists at the University of Bonn decided to look at viruses similar to hepatitis A in other creatures. They got samples from 203 species and discovered "highly diversified viruses in bats, rodents, hedgehogs, and shrews" — 13 novel Hepatovirus species in all.
Based on their analysis of the near-complete genomes of 9 of those hepatovirus species, the researchers concluded that the virus probably started out in an insect, and then jumped to the small mammals that fed on the insect. From there it jumped to humans. Which small mammal was the culprit? Bats. According to the study, "Antibodies in some bat sera immunoprecipitated and neutralized human HAV, suggesting conservation of critical antigenic determinants."
Don't worry. Bats aren't currently flying hep A vectors — just vectors for rabies — but the scientists believe that, back in the day, well before there was a large enough human population to keep hepatitis A chugging along, bats probably kept the virus alive until the human population grew and could keep infecting itself.