So the aneurysm itself is basically a weakening of the blood vessel that will often "pouch" out. You can have one and be basically fine, for a time. But if it ruptures, that is the problem, as it bleeds into the brain cavity.

Depending on where the vessel weakness occurs, if discovered early enough, it can sometimes be treated through endovascular treatments like coiling and pipeline stents. Diagnosis of an unruptured aneurysm is often incidental, sometime on a non related head ct scan. Unless someone has a strong family history of brain aneurysm, most people don't get regular head CTs because of the radiation exposure and cost.

Some come develop rapidly, and some gradually develop and worsen over much longer periods, so you would have to theoretically get scans fairly often to really screen well for them, though that comes with its own set of risks. Which is why they seeming come out of nowhere.

So technically, some are preventable. But until we get to the point where we scan everyone's head on a regular basis, it's still a pretty small number.

Source: Inpatient Neurosciences NP.