In computer virus terms, metamorphic code is code that can reprogram itself. Often, it does this by translating its own code into a temporary pseudo-code, and then back to normal code again. This is used by some viruses when they are about to infect new files, and the result is that their "children" will never look like them selves. The computer viruses that uses this technique does this in order to avoid the pattern recognition of the anti virus-softwares: Where the actual algorithm does not change, everything else might.
Metamorphic code is more effective than polymorphic code. This is because most anti virus-softwares will try to search for known virus-code even during the execution of the code.
Metamorphic code can also mean that a virus is capable of infecting executables from two or more different operating systems (such as Windows and Linux) or even different computer architectures. Often, the virus does this by carrying several viruses with itself, so it is really a matter of several viruses that has been joined together into a "supervirus."
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