One of the few mercies of the spreading coronavirus is that it leaves young children virtually untouched — a mystery virologists say may hold vital clues as to how the virus works.

In China, only 2.4 percent of reported cases were children and only 0.2 percent of reported cases were children who got critically ill, according to the World Health Organization. China has reported no case of a young child dying of the disease covid-19.

The new coronavirus has proved especially deadly on the other end of the age spectrum. The fatality rate in China for those over 80 is an estimated 21.9 percent, per the WHO. For ages 10 to 39, however, the fatality rate is roughly 0.2 percent, according to a separate study drawing on patient records of 44,672 confirmed cases.

That means the new coronavirus is behaving very differently from other viruses, like seasonal influenza, which are usually especially dangerous for the very young and very old.

“With respiratory infections like this, we usually see a U-shaped curve on who gets hits hardest. Young children at one end of the U because their immune systems aren’t yet developed and old people at the other end because their immune systems grow weaker,” said Vineet Menachery, a virologist at the University of Texas Medical Branch. “With this virus, one side of the U is just completely missing.”

“It’s the aggressive response from their immune system that is damaging them, even more than the infection itself,” Menachery said. “It’s like police responding to a misdemeanor with a SWAT team crashing through the door.”

The question he and others have still struggled to answer, however, is why the baby mice escape unscathed.

Some experts have floated a theory that because children are so heavily exposed to four other mild coronaviruses, which circulate every year and cause the common cold, that may give kids some kind of strengthened immunity. But many have doubts about that argument because adults catch the common cold coronaviruses too, and the immune systems of children — especially under the age of five — are underdeveloped, which should make them more vulnerable, not less.

“If it bears out that kids are less prone to infection, then I suspect there’s something more mechanical than immunological going on,” said Espers, the pediatric infection expert. “Something about the receptors in children’s bodies or their lungs is interfering with the virus’ ability to attach itself.”