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(INDIANAPOLIS) — We know senior citizens are more at risk from coronavirus. What we don’t know for sure is why.

Coronavirus has struck adults in Indiana more or less equally across different age groups, with the highest infection rate among people in their 40s. But the death rate is another story. Hoosiers over 70 make up three-quarters of the deaths, and the vast majority of the younger victims have underlying health conditions.

Most viruses hit seniors and babies hardest, because babies’ immune systems are still developing, and seniors’ are starting to wear out. But coronavirus has mostly spared the young. And when it does kill, it often does it by overstimulating the immune system, creating a “cytokine storm” which attacks cells indiscriminately instead of zeroing in on the virus.

IU Health infectious disease specialist Cole Beeler says it’ll take further research into the virus’s makeup to figure out why it affects age groups differently. But aging typically brings chronic low-grade inflammation, and Beeler says one theory suggests that inflammation may trick the immune system into misreading too many cells as unhealthy.