An antivirus is a program designed specifically to eliminate viruses. Unfortunately, viruses, in their many forms, don’t always fit the same mold. Even if they did, there are over a hundred thousand viruses out there. Over a hundred thousand. That is a lot of code designed to take advantage of thousands of different flaws, holes and problems to accomplish any number of things.
As was discussed earlier, not all viruses exist for the sole purpose of replication, and often they may not even exist as a ‘file’ (which is a prerequisite for scanning for most virus scanners). Many newer viruses, worms really, will never ever exist as a file on the target computer. They exist only in system memory and infect an application only while it is running. This prevents the antivirus from being able to disinfect the system, and can also provide the virus with the opportunity to disable, break, or corrupt the antivirus program, potentially creating an increased deception that the antivirus is continually effective even though it now answers only to the bidding of an anonymous connection across the Internet.