TOKYO—Shuichi Takatori spent two weeks in the hospital this fall with a fever after catching Covid-19. He recovered, went back to work and now feels fine.

But the disease lingered on in a different way—in the stigma Mr. Takatori says he felt from society. The 60-year-old member of Japan’s Parliament decided to disclose his illness, although he said he feared the consequences in next year’s election, and word quickly got around.

A restaurant where had dined called his office to ask for damages. The school basketball team of an aide’s child was disinvited from a tournament. And weeks after his recovery, he says relatives told him not to visit his hometown for a service on the first anniversary of his father’s death.

“I feel even now like it’s something embarrassing,” said Mr. Takatori, a member of the ruling Liberal Democratic Party.