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The spread of the coronavirus through a Seattle-area nursing home seemed to underscore a key point about the disease: Older and sicker individuals are most at risk.

And while it is true that nursing home residents are particularly vulnerable to serious complications from the illness, younger and middle-age adults, those in their 30s, 40s and 50s, are far from immune from catching the virus.

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Indeed, many of the cases across the U.S. have been reported in younger and middle-age adults.

In California, 246 cases have been diagnosed in adults under 64, while just 135 have been in people over 65. In Colorado, more than half of the cases have been diagnosed in people in their 30s, 40s and 50s. In Georgia, 46 percent of cases have been among adults under 60.

Other countries, too, have noted cases — including severe ones — in younger adults.

While 38 percent of patients in Italy have been over 70, 37 percent have been in their 50s and 60s, according to a study published Tuesday in the Journal of the American Medical Association. About a quarter are adults younger than age 50.

Simply looking at the age ranges of the infected, however, doesn't provide any insights into the severity of the illness.

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"In Italy, we're seeing a lot of younger adults get very sick," Dr. Peter Hotez, co-director of the Center for Vaccine Development at Texas Children's Hospital and dean of the National School of Tropical Medicine at the Baylor College of Medicine in Houston, told NBC News.

He said that while it's too soon to analyze how the virus will affect the majority of Americans, no matter their age, it's possible people in their 20s and 30s may develop severe illness.

In France, health officials said more than half of the country's 300 patients in intensive care units are under 60, The Associated Press reported.

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At Holy Name Medical Center in Teaneck, New Jersey, most of the hospitalized coronavirus patients are younger than 65.

"We're seeing younger patients, one a 29-year-old, some in their 40s, 50s. " Dr. Adam Jarrett, the center's chief medical officer, told NBC News on Monday, adding he was surprised patients' ages skewed younger than he'd expected.

All but one had underlying health conditions, such as heart or lung disease, diabetes and obesity.

"I am concerned about people who are obese," Jarrett said. "I wonder about lung capacity." COVID-19, the illness that results from infection from the coronavirus, is a respiratory disease that can lead to pneumonia.

Jarrett said that while many younger people are recovering, others have not been as lucky. "We are seeing organs starting to fail, such as kidneys," he said. Eight of the patients at Holy Name Medical Center are on ventilators and, so far, not showing signs of improvement.