COVID-19, the disease caused by the novel coronavirus, has been much deadlier in older people, but more anecdotes are popping up of young, healthy people getting critically ill. Among the first reported cases in the US, around 40 percent of the patients that required hospitalization were between the ages of 20 and 54, according to a new report from the Centers for Disease Control and Prevention (CDC).

Meanwhile, tragic stories of young people getting gravely sick and dying are starting surface. Two 29-year-old health care workers became seriously ill in Wuhan — only one survived, a report in The New York Times detailed. Other stories about people under 50 coming down with serious symptoms are making the rounds on social media, along with questions about whether seemingly healthy young people ought to be more concerned. What do these cases tell us about the disease?

A dear friend in Madrid is in a coma with Covid-19. He’s under 40 and was in excellent condition. So much for it only killing the old and infirm. Why are NYC bars and restaurants still open? — James B. Stewart (@JamesStewartNYT) March 15, 2020

There are reasons for people in every age group to be cautious, health experts say, but not because our understanding of who is most vulnerable to the virus is changing. Just because older people are more likely to develop a severe case or die from the disease, it doesn’t mean some young people won’t. That smaller number of severe cases among young people still doesn’t disprove the data that shows that seniors are the most vulnerable.

The CDC data highlights that the young are not immune to getting seriously ill, with 38 percent of hospitalized patients between the ages of 20 and 54. However, the data may not be entirely representative of what’s happening in the United States. The CDC report analyzed the first 2,500 reported cases in the US, and the first wave of cases is likely going to be the most severe since testing has been extremely limited in the country for some time. Patients who had the most prominent symptoms are the ones who got tested first, so it’s possible people with lesser symptoms went unnoticed. Still, the data confirmed what we’ve learned from other countries: the risk of dying from COVID-19 increases drastically with age, with 80 percent of deaths in persons over 65 years old.

The US will likely see more young people get really sick over the next several weeks simply because the pandemic is still growing. Days can pass before someone starts feeling the symptoms of COVID-19. So experts anticipate a jump in the number of cases in the US as those who have been in incubation periods start feeling ill and more people get tested for the virus.

“I think it’s almost a math problem in some ways, right?” says Benjamin Singer, an assistant professor of medicine in pulmonary and critical care at Northwestern University Feinberg School of Medicine. “Even with a lower likelihood of younger people getting critically ill, a certain percentage will get critically ill, and since the denominator is growing and growing these cases are popping up more and more.”

Underlying health conditions can also make younger people susceptible to a severe case of the virus. “Where you find severe cases or even death in young people, we don’t really have full information on these patients,” says Lee Riley, chair of the division of infectious disease and vaccinology at the University of California, Berkeley. It’s possible that some of those people might have had medical conditions that we just didn’t know about. Chronic conditions that can affect young people, like diabetes, can make it more difficult for someone to recover from the disease.

“We can’t do this without the young people cooperating.”

Even younger people who feel perfectly healthy need to take the pandemic seriously because they can spread the virus even if they’re not feeling any symptoms, Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases said. “Don’t get the attitude, ‘Well, I’m young, I’m invulnerable,’” Fauci said at a press briefing on the 17th. “You don’t want to put your loved ones at risk, particularly the ones who are elderly and the ones who have compromised conditions. We can’t do this without the young people cooperating. Please cooperate with us.”

Update March 19th, 12PM ET: This article was updated to include new information from the CDC.