"For over 25 years, the computer community has been dealing with malicious software. These little pieces of malware started as minor pranks and theoretical explorations into subverting security controls, but soon developed into more malicious programs that have left a significant impact on the computing community. Over the last decade we have seen viruses/Trojans/worms wreck countless computer systems, shutdown entire business, and impact our economy on a scale that rivals that of a major natural disaster.
Ironically, the solution that the industry came up with to solve this problem is little more than a band-aid. Viruses have gotten more sophisticated and have grown in number to the point where a user has to update their software daily to maintain a reasonable level of safety, and even that isn’t guaranteed to keep their computer virus free. In addition, since the anti-virus industry has moved to a subscription-based market, users have to shell out $30 a year for this less than perfect solution. So what exactly are you paying for? In this section we are going to take a look under the outer shell of the typical antivirus program and examine how they work — and how they fail." InformIT: Security Reference Guide > How Antivirus Programs (Don’t) Work