Computer virus is a piece of code that is capable of copying itself and typically has a detrimental effect, such as corrupting the system or destroying data.

• Conficker.
• Creeper virus - The first malware that ran on ARPANET.
• Leap - Mac OS X Trojan horse

WORMS

is a self-replicating virus that does not alter files but resides in active memory and duplicates itself. Worms use parts of an operating system that are automatic and usually invisible to the user.

• MSBlast.
• Code Red.
• Nimda.