In 2013, I was in ok shape, late 30's, running maybe about 15 miles/week with some gym activity. I was suddenly hospitalized for 4 days with pneumonia and "something else." About four months later, I got on a treadmill and couldn't run for more than a minute.

Long story short, I was diagnosed with viral cardiomyopathy. A virus basically comes in, fucks up your heart, then leaves. Myocarditis was the method in my case. They never ran a test to see what virus, because at that point, there wasn't any reason to. The damage was done. My ejection fraction was at 15% (50-55% is normal). I was given a defibrillator vest to shock me if I collapsed, and eventually I got a permanent implant.

In various conversations with cardiologists, they mentioned that it's believed to be much more common than they once thought. My primarily cardiologist said the week prior to me, she had a young, perfectly healthy male, in his early 20's that had a very similar hospitalization event. His was so bad they immediately put him on the transplant list.

I credit returning to long distance running. I don't recommend it as a treatment. Not scientifically studied and was/is definitely against doctor's orders.

Don’t underestimate flu

https://wwwnc.cdc.gov/eid/article/22/6/15-1080_article

Some of those cases do have heart problems.