Virus writers have adopted various strategies to fool anti-virus software. One of the most popular and successful ways of fooling anti-virus software is to create a stealth virus. Stealth viruses contain a code that inserts itself between the operating system and the anti-virus software.
Whenever a check is made for a virus, the stealth virus tells the operating system that there is no problem. In this way, the anti-virus software can be ignored and the virus can continue doing its work
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