A university cluster
Kyoto Sangyo University says four of its seniors returned from a tour of Europe on March 14, before the government began asking people flying in from Europe to self-isolate for two weeks.
Three of the four later tested positive for the coronavirus, but not before they had attended farewell parties, taken part in club activities, and then returned to their hometowns.
Authorities believe at least 40 infections across eleven prefectures can be traced to the seniors.
Experts warn against complacency
The World Health Organization is warning young people not to be complacent about the virus, saying that while most show only minor symptoms, some are falling seriously ill or dying. It also warns that even if young people have mild symptoms, they can still infect weaker people and seriously threaten their lives.
Kutsuna Satoshi of the National Center for Global Health and Medicine has treated more than 30 people infected with the new coronavirus. He says there is a risk of overconfidence among the young and urges them to avoid going out not just for their sake, but for the safety of those around them.
Virus expert finds his voice
A researcher from Kyoto University's Institute for Frontier Life and Medical Sciences has found a way to connect with a young audience. He began tweeting advice after it became clear that asymptomatic people can spread the virus. He said he felt scared that people have too little sense of crisis.
And he used language you wouldn't expect from an expert:
"While you're out, don't touch your eyes or nose. And never pick your nose. Or if you really have to pick it, wash your hands thoroughly first and then give it a really good rummage."
"When you're eating a meal with someone, shut up! Talk later when you've put your mask back on."
And he told people that, rather than trying to avoid catching the virus, they should focus on not transmitting it. "If you can't understand that, get the hell out of here," he tweeted.
His unusually coarse phrasing is resonating with young people. The tweets got tens of thousands of likes each.
The government is also trying to reach young people. The Ministry of Health, Labor and Welfare created a video featuring safety messages from 13 top female fashion models.
With infections now surging in Japan, the experts and authorities are trying anything they can to overcome the youthful sense of invincibility and convince teens and 20-somethings that everyone has to fight the virus together.