That's an excellent question and the fact that you are asking it is a sign that you are taking security serious.
People usually get hacked because they fail to stick to basic security behaviour, like using the same passwords in differens places, using dumb passwords, believing that virus checkers will always catch malware, or are talked into starting software or visit web sites of some kind, like for duping, maphacks or using bots.
Sometimes people invent these "I've been hacked" stories because they have been scammed in a silly manner or their account has been banned for game license violations and they don't want to admit to it.
So if you are cautious and don't believe anything just because you want to believe it, you era probably rather safe. If you still don't feel safe enough, you can use an authenticator in order to make bnet account access safer. See the Blizzard webpage for details.