The notion that coronavirus is “just a cold” or “no worse than the flu” for young people is proving to be untrue.

“The COVID-19 virus is capable of causing infection and severe disease in all people of all ages,” said Dr. Maria Van Kerkhove, head of World Health Organization’s emerging diseases and zoonosis unit, on Friday. “The data that we’ve seen from a number of countries is that the majority of children that are infected are experiencing mild disease,” she said, adding that there a handful of cases of children dying from coronavirus.

While there’s no question that the elderly and those with underlying conditions have been affected much more than other age group, younger people are not immune: 20% of deaths in Korea were people under the age of 60, and 15% of people in intensive care units in Italy were under 50 years of age, said Dr. Michael Ryan, WHO’s top emergencies expert.

In New York state, where there are now more confirmed coronavirus cases than in France or South Korea, nearly 54% of hospitalized coronavirus patients were between 18 and 49, Gov. Andrew Cuomo, a Democrat, said Saturday.

Among the latest fatalities: A 36-year-old public school principal from New York City — where there were more than 25,573 cases of coronavirus as of Friday morning — died from coronavirus complications on Monday, the New York Post reported. It was not known whether she had any preexisting conditions.

Only 8% of the cases of coronavirus in New York City are from people above 75, as of Friday morning, according to the city’s health department. However, 50% of the city’s 366 total deaths were people older than 75, and 97% of the deaths were people who had underlying conditions

Meanwhile, a teenager in Los Angeles may have been the first person under 18 to die from the coronavirus, but more testing was needed to confirm that, the Los Angeles Times reported, and public health officials in Philadelphia said that the majority of its 34 cases were people between the ages of 20 and 39.

A 44-year-old man from New Braunfels, Texas died on Thursday, two days after being diagnosed with coronavirus, the New York Post reported. The father of six was a kindergarten teacher.

Younger people are being hospitalized at unexpectedly high rates, particularly in New York State.

In the U.S., people under 44 make up 20% of hospitalized coronavirus patients, according to a report published by the Centers for Disease Control and Prevention last week. Patients under 65 accounted for nearly half of those admitted to hospital intensive-care units for COVID-19.

In Atlanta, a 12-year-old girl was placed on a ventilator and was fighting for her life after she was diagnosed with pneumonia brought on by a coronavirus infection. The girl, only known only as “Emma” due to privacy laws, had no preexisting conditions, her cousin, Justin Anthony, told CNN.

The CDC has not publicly reported the median age of coronavirus cases in the U.S. In California, the most populous state in the U.S., the median age is 47, the state’s health department reported last week: 42% of the state’s 1,733 cases of coronavirus are between ages 18 to 49.

The CDC’s report didn’t specify whether patients had underlying health conditions like obesity, diabetes and cancer which increase the likelihood of contracting COVID-19, the disease caused by coronavirus.

There have been cases among young people who were in peak physical health. Cameron van der Burgh, 31, an Olympic gold-medalist swimmer from South Africa, says he has been battling coronavirus for the past two weeks.

“Although the most severe symptoms(extreme fever) have eased, I am still struggling with serious fatigue and a residual cough that I can’t shake. Any physical activity like walking leaves me exhausted for hours,” he wrote on Twitter TWTR, -4.24%

“By far the worst virus I have ever endured despite being a healthy individual with strong lungs (no smoking/sport), living a healthy lifestyle and being young (least at risk demographic),” he added.“Please, look after yourself everyone! Health comes first — COVID-19 is no joke!” he added.

Van der Burgh and other millennials were believed to be less prone to developing serious health complications from coronavirus. But as the virus continues to spread in countries outside of China, where it originated, younger people are being hospitalized at unexpectedly high rates.

COVID-19, the disease caused by SARS-CoV-2, the official name for this new coronavirus, had infected at least 92,932 people in the U.S. by Friday afternoon, exceeding the number of confirmed cases in both China and Italy, and killed at least 1,380, according to data aggregated by Johns Hopkins University.

Worldwide, there were 558,905 confirmed cases and 25,336 deaths as of Friday afternoon, according to data compiled by Johns Hopkins University; 127,768 people had recovered.

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What’s behind the number of young people with coronavirus?

So what’s the reason behind these cases? The fast-paced lifestyles of some young people, eating habits in the U.S. and the number of young adults not practicing social distancing may help explain why early data in China differs from the U.S. and Europe, experts suggest.

Data from China, where the pandemic originated, suggested that people in their 70s and 80s were most likely to die from the disease, and as the coronavirus arrived in the U.S, health officials warned older people to take extra precautions to avoid infection.

“Disease in children appears to be relatively rare and mild with approximately 2.4% of the total reported cases reported [1,342 people] amongst individuals aged under 19 years. A very small proportion of those aged under 19 years have developed severe (2.5%) or critical disease (0.2%),” the World Health Organization reported last month.

‘We do have an issue with younger people who are not complying.’

Those figures were reassuring to many parents and young people, but they may also have led more young adults to believe they were virtually immune from coming down with more severe symptoms and, as a result, less likely to change their lifestyles to prevent the virus spreading.

“We do have an issue with younger people who are not complying,” Cuomo said last week, referring to social distancing efforts. “So you’re not Superman, and you’re not Superwoman, you can get this virus and you can transfer the virus and you can wind up hurting someone who you love or hurting someone wholly inadvertently.”

In Italy, where the number of deaths has surpassed the number in China, the median age of those who have died is 80. But as the virus continues to spread in countries outside of China, younger people are being hospitalized at unexpectedly high rates.

But that does not explain many of the severe cases among the young. Indeed, while age may be one of the most convenient factors to study, it can be misleading, said Dr. Gregory Poland, an infectious-disease expert and director of the Mayo Clinic’s Vaccine Research Group in Rochester, Minnesota.

Another piece of the puzzle: In the U.S., 18.5% (13.7 million) people between the ages of two and 19 are obese according to the CDC. That figure rises to 42.7% for all adults. When an obese person has difficulty breathing, their lungs won’t expand as much as a healthy person’s lungs, Poland said.

“You take an extremely healthy 16-year-old boy from South Korea who plays soccer and has a vegetarian diet and doesn’t do any drugs, and compare that to a 16-year-old obese type-two diabetic kid in the U.S. and you’re talking about two different ages.”

“No one ever heard of this virus prior to 11 weeks ago so we’re really building the plane while flying it,” Poland said. To complicate matters further, China does not have a history of transparency, he added. “I wouldn’t be surprised if we find out about more severe deaths in kids.”

In China, 51 is the median age of coronavirus cases, according to February’s WHO report. In Wuhan, the epicenter of the pandemic, 39 of the 45 designated hospitals there were reserved for patients either in severe condition or older than 65 years old.