[beat]

Keep in mind that there are health conditions that can kill seemingly healthy young adults instantly - aneurisms, heart attacks, brain cancers, and others.

I'm reminded of one of my dogs dying of hemangiosarcoma - a cancer of the blood vessel linings that thankfully only happens to dogs, not humans. In many cases, the way we find out a dog has hemangiosarcoma is that the dog suddenly just drops dead. (In Rubu's case, it was three weeks from the first symptoms.) There are similarly fast and deadly cancers in humans.

[TillE]

Yeah. Deaths from undiagnosed heart conditions make the news every so often when it happens to young athletes. Sometimes very young. I believe just in the past few years, defibrillators have become standard equipment at football grounds in Spain and England.

[sliverstorm]

Undiagnosed conditions aside, there is also commotio cordis, which I believe has killed a number of high school sportsmen.

[rytis]

Commotio cordis (Latin, "agitation of the heart") is an often lethal disruption of heart rhythm that occurs as a result of a blow to the area directly over the heart (the precordial region), at a critical time during the cycle of a heart beat causing cardiac arrest. It is a form of ventricular fibrillation (V-Fib), not mechanical damage to the heart muscle or surrounding organs, and not the result of heart disease. The fatality rate is about 65%. https://en.wikipedia.org/wiki/Commotio_cordis

[Turing_Machine]

Yes. A girl in my high school died of a brain aneurysm right in PE class. Perfectly healthy, by all appearances, then... gone, like turning out a light. As I recall, they said that she'd likely had it since birth.