Computer Infections can be broken up into 4 main categories which are explained in detail below:

Viruses
Computer Viruses are small pieces of software that attach themselves to real programs. An example would be a virus that attaches itself onto windows explorer. Each time you run the program windows explorer the virus will also be executed and perform the function that it was meant to perform. This can be several things such as damaging files on your computer, or erasing them all together. This is also the time in which the virus will try to spread itself onto other programs and even other computers!

Worms -
A worm just like a virus is a small piece of software. However worms rather then relying on an external program to function, will run themselves over computer networks and security holes to spread. The worm will scan the network from it's host computer until it finds another computer that it can attach itself to. It will then continue this process to replicate. Because this type of infection runs by itself it can have devastating impacts. The Mydoom worm is estimated to have infected over a quarter million computers in a single day when it first launched back in March 1999.

Trojan Horses
A Trojan horse is a computer programs that claims to be one thing, but is actually another. For example you might receive a Trojan horse that is disguised as an image, but when you go to open the file, it will do whatever it is that it was written to do.

E-mail Viruses
Email Viruses are very common. They work by spreading themselves through email messages. Once they infect a computer, they will then go ahead and replicate themselves by automatically mailing out again to everyone in the victim's e-mail address book. Several of these virus's have raced through the internet. Examples of email virus's are the Melissa Virus, and the ILOVEYOU virus.

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About the Author
Ryan Fyfe is the owner and operator of Software Area. Which is a great web directory and information center on Anti Virus Software and other computer software related issues.