The ending reveals the source . . .
https://www.youtube.com/watch?v=5UkXOj8u1Fo
However:
At the end of the movie, with ominous music in the background again, we see a bat grab some fruit and drop it to the pigs below. We see the pig get sold and prepared by the chef that holds hands with Paltrow’s character for a photo. And that is presumably how it all started. In real life, it would likely take several contacts between animals and humans before a virus becomes fully transmissible to humans. It’s also unlikely that there was enough virus on Paltrow’s character’s hands to spread and infect other people. -- https://thehill.com/changing-america/well-being/prevention-cures/481652-what-does-the-movie-contagion-tell-us-about