As anybody who uses a computer these days knows, there is a lot of harmful software out there. The terminology is a bit confusing: adware, spyware, viruses, trojans, worms, rootkits, exploits, redirects, hijacks, to name a few of the more common forms of malware. What the hell is this stuff, how does it get on your computer, and how can you get rid of it?

Malware (“malicious software“) has been around almost as long as computers have. But it has changed drastically changed over the years. Once upon a time, most malware was basically digital vandalism, whose purpose was to wreak general havoc, as well to call attention to its creator and show off his programming skills. The programs were simple, easily defeated, and generally speaking, fairly harmless. But with the rise of the Internet, criminals became aware of the vast possibilities for making money using various forms of malware. The schemes have gotten progressively more sophisticated as the stakes have risen. Now, the making and distribution of malware is a multibillion-dollar business, controlled by international criminal syndicates. It is a virtual arms race, and the end is nowhere in sight.

“How DOES this happen?” I often hear. Most malware arrives via the Internet, as you might imagine. Viruses typically arrive in infected email attachments, or buried in programs downloaded from non-legitimate sites. Much of the malware that travels via infected email practically announces itself; it comes from someone you don’t know, or have no reason to hear from. The subject line doesn’t quite make sense or is poorly written, contains an unseemly invitation, or demands that you do something RIGHT NOW. And there is always an attachment. It’s the attachment that contains the tainted payload. As long as the attachment is unopened, nothing happens. Viruses and worms are spread in this way. A virus replicates by infecting executable programs on the host computer, whereas a worm propagates from computer to computer across a network connection, typically in a work environment.

Viruses and worms require some action by a user to spread. But more and more, malware is delivered surreptitiously, without any overt action by the recipient. NEXT Stealth Infections: Trojans, Rootkits, and Backdoors.