What causes a massive heart attack or stroke?

Heart attack : Blockage of coronary artery causes heart attack, and blockage of carotid artery causes stroke. There are many other possible causes.

Atherosclerosis: Depends on one's personal risks smoking, obesity, hypertension, hyperlipemia, diabetes , family history. Simply, one blocks the feeding artery to part of the heart muscle and low flow, or no flow causes ischemia which cause death of the heart cells, which is a heart attack! myocardial infarction is the medical term.Ekg and lab tests show the damage.

PlaqueRupture ; Clot: Atherosclerosis, typical onset -age 7, yet asymptomatic for decades (arteries thicken ; enlarge; not narrow), ; eventual plaque rupture triggering blood clot in artery opening which both sheds downstream ; becomes large enough to narrow ; close artery opening (both blocking blood flow) causes both the damage ; residual narrowings.