What Causes Brain Aneurysm?

hey my girlfriend just found out she has a brain anyresum 5MM, My question is, Whats her chance of survival?, I know i may worry to much BUT the doctors told her to come back in 2 days and they will tell her wether or not if she needs surgery or just take blood thinners and get it gone, I am really worried about her and was just wondering has many people survived a brain anyresum or is it as bad a cancer?