Researchers are confident it is zoonotic, and have traced it back to *likely* bats-to-human and *possibly*, additionally, pangolin-to-human transmission.
But those infected were very likely completely unaware they were sick and were spreading the virus well before they had any symptoms. And, at that point, no one would have know they were transmitting the virus as it was/is a novel strain of coronavirus. For all most would have known, they had the flu, and would have taken normal (though we now know relatively ineffective) precautions.