Stealth Viruses

When a virus infects a computer it has to change something.  When a change in the computer is detected, usually so is the virus.  Stealth viruses attempt to cover their trail as they infect throughout a computer.  

All stealth viruses must be memory resident in order to work.  When a stealth virus infects, it take over the system function that read files or system sectors.  When something attempts to access the corrupted file, the stealth virus reports that the original file is there.  In reality, the original information is gone but this keeps the virus hidden.

Stealth viruses are best detected by booting from a disk that is known to be clean.  This way the virus is not memory resident when an anti-virus program is run.

Examples of Stealth viruses:  The first stealth virus was Brain written in 1986 which was a boot sector virus.  Frodo is another stealth virus that is a file virus.