How does an antivirus software find out whether a file is a virus or not?

I opened a USB drive today and my AVG popped up - THREAT DETECTED.

i wonder what is the logic behind the functioning of an antivirus software? how can it scan a pc so quickly and find so many different types of virii?

How does an antivirus software find out whether a file is a virus or not?computer security

Every virus has a virus signature, much like a finger print. A signature is a unique string of bits, or the binary pattern and virus scan software use this signature to detect and identify specific viruses. it scan through the files and keep comparing it to the signature files (usually dat file) it has. Once a virus signature has been detected, it will flag it as virus. Some viruses share the same signature.

http://antivirus.about.com/od/whatisavir...

How does an antivirus software find out whether a file is a virus or not?free spyware remobal

A virus is a program that copies itself to the memory and tries to copy itself to other application that are open.

Viruses can come from

1)Email attachments

2)Floppies(if used previously in a infected computer)

3)Pen drives (if used previously in a infected computer)

4)etc.

Viruses basically destroy the system files in the computer so that you have to buy a new one.