system
Given the following post, respond as if you're commenting on Reddit:
human
Why do bats make such good reservoirs for zoonotic viruses?
gpt
The main factor is that they are extremely social, living in colonies with millions of individuals while also traveling long distances and interacting with other colonies. They move back forth between colonies picking up and passing around diseases giving them a lot of opportunity to evolve. 

Pig pathogens are far more likely to be able to infect people but pigs don't get around nearly as much as bats, if a new pathogen infects one pig farm it will take a lot less time for it burn through the population.