Greta Thunberg, the 17-year-old Swedish climate activist, announced on Tuesday that she and her father, Svante, had symptoms of Covid-19 and that while hers were mild, it was “extremely likely” that she had contracted the virus. She used the announcement to urge young people to stay at home, even if they don’t feel sick, to protect those who are more vulnerable.

“Many (especially young people) might not notice any symptoms at all, or very mild symptoms,” she said on Instagram, where she has 10 million followers. “Then they don’t know they have the virus and can pass it on to people in risk groups.”

“We who don’t belong to a risk group have an enormous responsibility, our actions can be the difference between life and death for many others,” she said.

Ms. Thunberg spoke to European Union lawmakers at a meeting in Brussels in early March. In an effort to protect her mother and her sister at home in Stockholm, Ms. Thunberg said she and her father, who accompanies her on her travels, had isolated themselves in a separate apartment.