A stroke happens when the brain is deprived of oxygen and nutrients that is normally supplied through blood. So a stroke is caused by anything that interrupts the blood flow through the brain. The two most common ways this happens is when a blood vessel is blocked (ischemic stroke) or when a blood vessel bursts (hemorrhagic stroke). Of course, extreme undernutrition and respiratory failure could lead to an inadequate supply of nutrients and oxygen to the brain, but at that point, you have other things you worry about.
These account for 85% of all strokes. They happen most commonly when the lining of the vessels is damaged in some way (e.g. from cigarette smoke) and then some fat can accumulate under it. This would slowly narrow the blood vessel. At some point, the lining becomes very weak and a slight interruption can lead to the formation of a big blood clot which would block the blood vessel completely. The part of the brain that depended on that blood vessel would die (if not treated quickly).
What could cause these? Like I mentioned, cigarette smoke (and diabetes) could damage the blood vessels initially, obesity or high levels of lipids (like cholesterol) would lead to the build-up of fat which would narrow and weaken the blood vessel until it clots eventually.
Other forms of ischemic strokes could happen when a blood clot at some other place in the body forms, then a tiny bit of it breaks up, travels in the bloodstream, and lodges in a blood vessel in the brain. These are called embolic strokes. The most common cause of these would be heart conditions such as atrial fibrillation, which is why people with atrial fibrillation tend to be on lifelong anticoagulants (“blood thinners”) such as warfarin/Coumadin to prevent the formation of blood clots in the first place.
If the blood pressure in the whole body is too low (i.e. shock), then the brain couldn’t be nourished effectively.
Basically, these are just bleeds into the brain (or spaces surrounding the brain). They account for the other 15% of strokes. They’re commonly caused by head trauma which could burst the vessels directly. Other causes are very similar to the ones causing ischemic stroke, such as diabetes, smoking, obesity/dyslipidemia because all of these tend to weaken the blood vessels making them more prone to rupture. Hypertension (high blood pressure) could be severe enough to burst the vessels directly too (the higher the pressure, the more likely they are to burst). And some conditions are out of hand, such as having genetically abnormal vessels, amyloidosis, predisposition to forming aneurysms (there are some reversible risk factors though), etc…