I've read that the ebola virus was either

1. transmitted from eating monkey meat (it is very similar to a disease in monkeys; maybe that was HIV) or
2. because of diggin up ground that had been covered by jungle for 1,000's of years exposing things that hadn't been exposed for centuries.
3. Same thing in Siberia, permafrost is thawing, exposing dirt with microbes that are HUNDREDS of thousands of years old, which no life today has any resistance to, and also Anthrax, which is one of the most deadly