1. Viruses

2. A virus is a computer program that operates by piggybacking on other programs. Any time the other program is run, the virus is activated and able to do harm.

3. Any time the other program is run, the virus is activated and able to do harm.

4. Email addresses often replicate themselves and send emails to anyone on your address list. A famous example of an email virus is the Melissa virus - the fastest spreading virus ever. When the attachment was open, the virus automatically spread itself to the first 50 emails in the address book. The most amazing thing is, it was only because of human error in opening the attachment that it spread in the first place.

5. Worms

6. Worms spread through computer networks.

7. They use the network to replicate from machine to machine. One famous example of a worm is the Code Red worm. It "replicated itself over 250,000 times in approximately nine hours on July 19, 2001"

8. Trojan Horses

9. These are programs that claim to do one thing, such as a game, but when run secretly do other things.

10. Trojan horses can erase your hard drive for example.

11. However, they cannot replicate themselves. They have to be sent or copied by a user.

12. One famous Trojan horse program was the BackOrif ice. Computer users did not even realize they had downloaded a Trojan horse program.

13. However, it allowed the designers a "back door" into their computer. This meant they could easily do what they wanted without the user ever knowing it.