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
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.
> 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...