Incorrect. It's statements like that that make me dislike smug Mac-users >.<
We Linux users can be affected by a virus just like anyone else. The key difference is that the kind of virus that can cause real damage to a Windows computer (deleting key system files, for example) can do relatively little on a Linux machine due to the way security and user accounts work. Unless you log in as root all the time, the only things that can be damaged are a user's own files (which is still dangerous, but nowhere near as dangerous as losing something like fstab).