[Fai Mao]

I think it was carried out by rats through sewage pipes into the wet market next door.

I cannot prove that but having lived in Wuhan, I’d say it is a good guess.

[SaveFerris]

Then, their containment protocols were as lax as could be

[KarlInOhio]

Or an enterprising lab employee wholesaled dead lab rats to the wet market. No reason to toss them in the incinerator rather than the wok.