Stealth viruses are those written in a way so as to avoid detection by standard anti-virus software. A particular virus may be a file virus and a stealth virus, or a boot-sector virus and a stealth virus. The techniques used to avoid detection are self-encryption (where the virus converts its code into some sort of program code), and alteration of disk directory data. One example of a stealth virus is the Whale virus. This is, both, a file infector and a stealth virus. This virus infects .EXE files and adds 9216 bytes to each file size. The virus then subtracts 9216 bytes from this file's entry in the directory to give the impression that no change has taken place.
