I'd further note that the H1N1 flu is widely believed to have gone extinct in the wild, then been reintroduced in 1977 from a laboratory source, probably through a failed vaccine trial:

> The reemergence was probably an accidental release from a laboratory source in the setting of waning population immunity to H1 and N1 antigens.19,31

https://www.nejm.org/doi/full/10.1056/NEJMra0904322

This explanation is now printed uncontroversially in major journals, though at the time the WHO rejected it:

> Laboratory contamination can be excluded because the laboratories concerned either had never kept H1N1 virus or had not worked with it for a long time.

https://pmc.ncbi.nlm.nih.gov/articles/PMC2395678/pdf/bullwho...