Virus scanners work in several ways. The most common is pattern
matching, where the scanner looks inside files for a string of bytes which
match those in its database of known viruses. This will find the majority
of viruses.
To detect viruses which use techniques such as polymorphism to
change their code slightly each time they infect, scanners also have to
use more sophisticated inspection techniques. These include processing
files in a real-time emulation mode to watch for the polymorphic engine to
decrypt the virus.
Most virus-scanner developers constantly improve their scanners to keep
up with the growth of viruses and the ever-expanding range of infectable
objects to inspect.
For more information on this topic, visit these other searchSecurity resources:
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David Strom's Security Tool Shed: myCIO.com Virus Scan ASAP
Scheier's Security Product Roundup: Antivirus tools: Unify and conquer
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