14.1. The Varieties of Malware
In everyday speech, the word virus is often used as a catchall term for any kind of malware. But, in fact, a virus is something very specific and, on a Mac, very rare. It's not even necessarily the most destructive type of malware, despite the ominous associations of its name. Because the risks and the methods of protecting yourself vary tremendously from one kind of malware to another, it pays to understand a bit about the kinds of malware you could encounter.
As you review this list, keep in mind that it's not exhaustive — new kinds of malware and variants of old ones crop up all the time — and that a given piece of malware may fall into more than one category.
In the simplest terms, a computer virus is a tiny, self-replicating program. Viruses aren't stand-alone programs like ordinary applications. Instead, they secretly attach themselves to files and applications on your computer. When you open an infected application (or certain types of infected files that include the capability of executing code, such as PDFs), the virus has the opportunity to run, and as it runs, it tries to copy itself into other files and applications — on your computer or on any available media (which may include network volumes). Those copies, in turn, also make copies of themselves.
Apart from copying themselves into files on network servers to which you're connected, viruses can't, of their own accord, move from computer to computer. They're ordinarily ...