No previous conditions doesn't mean no conditions
Lots of young people have unknown heart conditions. That's why CRY was set up because someone died young during exercise, and they now do free screening of youngsters (well anyone under about 35 I think).
Also doing lots of exercise lowers your immune system if you do too much, which could have been why the Italian marathon runner was hit so hard despite being so fit. Our running club has been telling us not to cut right back on running at the moment to protect our immune systems. I don't do that much anyway so I've cut back from about 20 miles a week to about 13, but some of the keener bods do 50, and they need to cut back a lot while this is going on.