The World Health Organization (WHO) warned that young people who have the coronavirus and don’t know it are the cause of the recent rise in virus cases.

“This increases the risk of spillovers to the most vulnerable: the elderly, the sick, people in long-term care, people who live in densely-populated urban areas and under-served rural areas,” Dr. Takeshi Kasai, WHO regional director for the Western Pacific, said during a press briefing on Tuesday.

These young people, who are in their 20s, 30s, and 40s, may never develop coronavirus symptoms or have mild symptoms, Kasai continued. This is further complicated by young people failing to follow social distancing guidelines, as has been witnessed in the U.S. by photos and videos posted to social media of large college parties.

Kasai called on world leaders, as well as the public, to “redouble their efforts” to prevent the spread of the virus to those people that are most at risk.

“We are encouraged to see many countries in the region already adopting new tactics that help minimize the social and economic consequences of COVID-19 and show how we can cope with it for the foreseeable future,” Kasai said.

Data published by the WHO also indicated that of the 6 million positive cases of the coronavirus it analyzed from Feb. 24 to July 12, it found that the number of positive cases of the virus for people aged 15 to 25 rose from 4.5% to 15%, Reuters reported.

Globally there 21.9 million positive cases of the coronavirus with over 774,000 COVID-19 deaths, according to Johns Hopkins University.

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Kevin McKeown of Black Pistol Fire during Day 2 of Music Midtown Festival at Piedmont Park on September 16, 2018 in Atlanta, Georgia. Getty Images/Scott Legato