This was a shock, since frontrunner John Fetterman is relatively young (52) and has an aura of invincibility. Via CBS News:

PITTSBURGH (KDKA) -- Pennsylvania Lt. Gov. John Fetterman is recovering following a stroke, his campaign announced Sunday. Fetterman, who is running for the Democratic nomination for the state's U.S. Senate seat, did not suffer "any cognitive damage," a release from his campaign said. He went to the hospital on Friday after he "wasn't feeling well."

"I had a stroke that was caused by a clot from my heart being in an A-fib rhythm for too long. Fortunately, Gisele spotted the symptoms and got me to the hospital within minutes. The amazing doctors here were able to quickly and completely remove the clot, reversing the stroke, they got my heart under control as well. It's a good reminder to listen to your body and be aware of the signs," the release said.

Fetterman said he is feeling "much better" and on his way to a "full recovery." "They're keeping me here for now for observation, but I should be out of here sometime soon. The doctors have assured me that I'll be able to get back on the trail, but first I need to take a minute, get some rest, and recover," Fetterman said in the release.