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I can't speak for all cases but there are often unknown medical conditions involved. My aunt got the flu one year and it weakened her heart so much that she required a transplant and while she lived for a while after that, she died at 27. Please vaccinate yourselves and loved ones even for the flu.
A friend of mine runs a charity [1] that deals with providing AED (Automated External Defibrillator) devices to organisations. These things save lives. They can deliver shocks to the heart if necessary and can guide laypeople through giving CPR until help arrives (I think many models now actually tell you if you aren't giving strong enough compressions, for example). I would also highly recommend people go through th…
So don't expect miracles where none are currently possible. Do all you can obviously in case of need, and ideally make yourself familiar with procedures a bit, any delay will worsen the chances.