Coronavirus can sicken or kill young people as well and they must also avoid mingling and spreading it to older and more vulnerable people, the World Health Organization said on Friday, even as photos of American students partying during spring break triggered outrage on social media.

With more than 210,000 cases reported worldwide and a death toll of 9,000, each day brings a "new and tragic milestone," WHO Director-General Tedros Adhanom Ghebreyesus said.

"Although older people are hardest hit, younger people are not spared. Data from many countries clearly show that people under 50 make up a significant proportion of patients requiring hospitalization," Tedros told a virtual press conference.

"Today I have a message for young people: You are not invincible, this virus could put you in hospital for weeks or even kill you. Even if you don't get sick the choices you make about where you go could be the difference between life and death for someone else," he said.

But for the first time the central Chinese city of Wuhan, the outbreak's epicenter, reported no new cases on Thursday, "providing hope for the rest of the world that even the most severe situation can be turned around," Tedros said.

The city of Wuhan registered no new cases of COVID-19 in 24 hours -- for the first time since reporting its first case in December in an outbreak that has gone on to infect more than 250,000 people around the world and kill more than 11,000 people.

"Of course, we must exercise caution; the situation can reverse. But the experience of cities and countries that have pushed back this coronavirus gives hope and courage to the rest of the world," Tedros added.

China as a whole is now reporting only a handful of new infections each day -- all of them apparently from overseas visitors -- as the crisis has shifted from Asia to Europe, which has now reported more deaths than China.

Tedros said the WHO's greatest worry was the impact that the virus could have if it took hold in countries with weaker health systems or more vulnerable populations.

"That concern has now become very real and urgent," he said, but added that significant sickness and loss of life in such countries was not inevitable.

"Unlike any pandemic in history, we have the power to change the way this goes," he said.

Amid global shortages of protective gear for health workers and diagnostic tests, Chinese producers have agreed to supply the WHO, Tedros said. Arrangements are being finalized and shipments coordinated to restock its Dubai warehouse to ship supplies where they are needed most, he added.

"Air bridges" will be needed to expedite supplies to countries for vital health workers, as many regular flights have been cancelled, according to Dr. Mike Ryan, the WHO's top emergency expert.

The WHO has distributed 1.5 million lab tests worldwide and it may need potentially 80 times that for the pandemic, he said.

Ryan, asked about Iran -- which is celebrating the Persian New Year as it battles the coronavirus which has killed more than 1,400 people and infected nearly 20,000 there -- said that such celebrations need to be modified.

Mass gatherings "cannot only amplify the disease but they can disseminate the disease very far away from the center," he said. "So they can be very, very, very, very dangerous in terms of epidemic management."

The WHO has shifted to recommending "physical distance" instead of social distancing to help prevent transmission of the virus, officials said.

"We are changing to say 'physical distance' and that's on purpose because we want people to remain connected," said Dr. Maria Kerkhove, a WHO epidemiologist.

"So find ways to do that, find ways through the internet and through different social media to remain connected because your mental health going through this (pandemic) is just as important as your physical health," she said.

(With inputs from AFP and Reuters)