Something not very mysterious —
2022 "Wildlife trade is likely the source of SARS-CoV-2"
https://www.science.org/doi/10.1126/science.add8384
2003 "Asymptomatic animal traders prove positive for SARS virus"
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1140695/
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The scary part is that there's every reason to expect this to happen again and again.