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An aneurysm is basically a swelling in the wall of a blood vessel, either because of build up within the vessels or weakness in the walls. The reason they are fatal is because this build up can sometimes burst leading to other complications and possibly death. Here is a link which may provide some info on aneurysms and some possible causes.
https://www.heartcenteronline.com/The_Aneurysm_Center.html
Offtopic but, This topic should probably be moved to the Health and Medical issues board |