By Alan Mozes

HealthDay Reporter

THURSDAY, April 9, 2020 (HealthDay News) -- COVID-19 has infected over 429,000 Americans and claimed the lives of nearly 15,000 patients, with seniors clearly bearing the brunt of severe disease.

But experts warn the virus is proving to be a threat to some younger Americans as well.

According to the U.S. Centers for Disease Control and Prevention, that's been true since Americans began getting sick. In mid-March, the CDC reported that patients aged 20 to 64 accounted for 20% of all COVID-19 fatalities, though more recent CDC statistics suggest the number may be more in the range of 10%.

Either way, the risk to young Americans is significant. Which begs the question: What exactly is going on?

It's impossible to know for sure, cautioned Dr. Panagis Galiatsatos, an assistant professor of medicine in the department of pulmonary and critical care medicine at Johns Hopkins University in Baltimore.

"I would strongly emphasize that this is a new virus, and we're learning about it in real time," he said. "And that means that whatever you hear today may be completely different by tomorrow."

So far, the main driver of serious illness among all patients, young and old, appears to be an immune system gone into overdrive.

It's called a "cytokine storm," Galiatsatos said. When it happens, the immune system keeps working, but at a potentially lethal level.

"If you think of your immune system as an orchestra, the more in harmony it plays the better the outcome," he explained.

"Sometimes we call this the 'Goldilocks theory," explained Jamie Sturgill, an assistant professor of internal medicine with the Division of Pulmonary, Critical Care and Sleep Medicine at the University of Kentucky in Lexington. "In that the immune system needs to work 'just right' to keep us healthy without causing disease itself," she said.

"But when the immune system starts to react to the virus in a chaotic way that harmony is out the window," Galiatsatos said. "And that's what a cytokine storm is."

The end result is "a very dire situation in which it's not only the virus that gets attacked, it's also your healthy cells as well," he said.