These five moments all come from one man's cardiac history, and prevention acts at four of them. Put them in the order they happened.

1. The town rebuilds its main road with cycle lanes and safe crossings
2. A routine check at his GP surgery flags raised blood pressure
3. He is started on medication to bring the pressure down
4. He has a heart attack on a Tuesday morning
5. A supervised exercise programme begins six weeks later

Cycle lanes, then the flagged reading, then the medication, then the attack, then rehabilitation.

Laid out on a life, the three levels stop being categories to memorise and become a matter of timing. The cycle lanes are primary prevention - they act before there is any disease, on a whole population, and the man never knows they helped him. The blood-pressure check and the medication are secondary: the disease process has started silently, and the point is to catch it before it announces itself. Rehabilitation is tertiary, and it begins only once the damage is done.

The attack itself is the pivot. Everything to its left is an attempt to stop it happening; everything to its right is an attempt to limit what it costs. That is the whole distinction, and it is a question of WHEN you intervene rather than of what the intervention looks like.