Stealth virus

Definition
A stealth virus is

"a resident virus that attempts to evade detection by concealing its presence in infected files. To achieve this, the virus intercepts system calls which examine the contents or attributes of infected files. The results of these calls must be altered to correspond to the file's original state. For example, a stealth virus might remove the virus code from an executable when it is read (rather than executed) so that an anti-virus software package will examine the original, uninfected host program."