Combined with reports from China of similar cases, the case in Japan has raised some uncomfortable questions. Reinfections are common among people who have recovered from coronaviruses that cause the common cold.

But those pathogens are very different from the new coronavirus, and experts said it’s unlikely that these are cases of people getting infected a second time.

“I’m not saying that reinfection can’t occur, will never occur, but in that short time it’s unlikely,” said Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai in New York.

Even the mildest of infections should leave at least short-term immunity against the virus in the recovering patient, he said.