Yes. And the malware could be polymorphic. Or there could be multiple versions of the same "core" out there. It's not clear to me how sophisticated virus (malware) scanners for OS X are with dealing with that.
From what I know (which is not much) scanners, among other things, search for identifying patterns in files. So there is an identifying pattern of each discovered malware/virus in a database.
lm2s|9 years ago