Mac® Security Bible

14.2. Macs as Malware Carriers

We've all heard of cases where someone is infected with a virus or bacteria and, although they don't get sick themselves (perhaps because of an especially strong immune system or a genetic anomaly), they instead act as carriers, passing on the illness to other people. In the digital world, the same thing can happen. A Windows virus or other malware can appear on your Mac but be unable to run or cause any damage because it wasn't designed to run on Mac OS X. However, if the malware finds its way from your Mac to someone running Windows, it can cause damage on their machine — and spread farther from there.

You may have noticed that I used vague, wishy-washy expressions such as "appear on your Mac" and "find its way" to another computer. But, in fact, any malware that's unable to run on your Mac is also unable to appear or spread by itself. So, what I'm really saying is that you could (presumably without realizing what you're doing) download malware, and you could then take some action that moves or copies it to someone else's computer. It can't "just happen," but you can, through ignorance, make it happen.

How might you do this? The most common way, by far, is to receive an email message containing a malware program as an attachment and then forward it to someone else. It probably goes without saying that email chain letters, get-rich-quick schemes, petitions, jokes, and other such messages that beg to be forwarded are likely sources of such malware. ...