Other polymorphic viruses vary instruction sequences and use false commands in the attempt to thwart anti-virus software. The process by which a virus makes copies of itself in order to carry out subsequent infections. Replication is one of major criteria separating viruses from other computer programs. A resident virus loads into memory and remains inactive until a trigger event. When the event occurs the virus activates, either infecting a file or disk, or causing other consequences.
All boot viruses are resident viruses and so are the most common file viruses.
Stealth Virus Stealth viruses attempt to conceal their presence from anti-virus software. Other viruses hide the actual size of an infected file and display the size of the file before infection. Stealth viruses must be running to exhibit their stealth qualities. Usually malicious action triggered at a specific date or time. Trojans are not viruses since they do not replicate, but Trojan horse programs can be just as destructive.