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polymorphic virusA virus that changes its binary signature each time it infects a new file. Using a random number generator, a unique key can be created to encrypt each virus download that self extracts and executes when opened. Without an identifiable pattern to match, the only way to discover a polymorphic virus is by its actions (see behavior detection). See self-extracting file.
Also called a "stealth virus," hundreds of thousands of polymorphic Windows viruses are discovered every day. The actual number of antivirus families that generate that many files is perhaps a few thousand. See virus and antivirus program.