Swine flu came from domestically raised pigs. Factory farming makes it more likely because the pigs are kept in close, confined quarters, increasing the spread between animals. The virus is passed from the animals to the farm workers.

Not this specific outbreak, but it was from the meat trade (wild caught or farmed). We keep stressed out animals in disgusting conditions in close proximity which is a breeding ground for disease. We then interact with them.

And of course, wild animals (like bats) can transmit diseases to other species (like pigs). Who can then transmit it to all the other pigs around it.

Then we pump them full of medication so they don't get sick, and end up creating antibiotic resistance.