The SARS outbreak of 2002 was caused by a bat coronavirus in China that infected some kind of wild mammal before infecting humans. Among the top suspects for that intermediate animal: the fluffy raccoon dog.

“You could not get a better textbook example of disease emergence waiting to happen,” Dr. Holmes, 57, said in an interview.

The tall, bald Englishman did his best not to draw attention to himself as he snapped a picture of the raccoon dogs, which look like long-legged raccoons but are more closely related to foxes. He then took a few more pictures of other animals in cages of their own. As a vendor began clubbing one of the creatures, Dr. Holmes pocketed his phone and slipped away.

The photos faded from his mind until the last day of 2019. As Dr. Holmes was browsing Twitter from his Sydney home, he learned of an alarming outbreak in Wuhan — a SARS-like pneumonia with early cases linked to the Huanan market. The raccoon dogs, he thought.

“It was a pandemic waiting to happen, and then it bloody well happened,” he said.