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WHAT IS A VIRUS?
When you hear about a computer virus, what comes to mind? Most people imagine something that gets on your computer and deletes everything. While there are definitely viruses that will do that, they are not the most common type today. Most viruses today either compromise the security of your system so it can be used to send out spam or just spread wildly. The ones that spread wildly might seem harmless to you, but they use up massive amounts of bandwidth, which costs your internet service provider (ISP) a lot of money. In the long run, this means higher prices for you.
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 Spam is the common term for unsolicited commercial e-mail. Prevention of this will be addressed in another Michael's Guide.
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You will often hear the terms worm or trojan used in place of virus. For the purposes of preventing them, you can consider them all the same. The only difference in them is the way they behave. Viruses infect a specific existing file. Worms spread without infecting a particular file. Trojans pose as something desirable but are actually a malicious item. Today, most of the "viruses" you hear about are usually worms.
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HOW DO VIRUSES SPREAD?
You can still get a virus the old fashioned way by opening an infected document. Macro viruses infect files such as Microsoft Word or Excel files. They come to life them you open the file on your computer. You could bring this file to your machine via a floppy disk, CD, e-mail, etc. These have become less common because of options in the programs that let you block them.
A virus can also spread over a network. One infected computer connected to a network at home or work can spread certain viruses to the other machines on the network without you realizing it. This also means that re-infection is a serious possibility if you miss one machine and don't keep your other machines properly protected. A company that I once worked for spent a lot of time and money cleaning the Klez worm from its computers. I was hired to come in and remove it from all of their computers again because they had missed a few the first time around and before they knew it they had machines in two states that were re-infected.
The last method is the most common. It's e-mail! You receive an e-mail from your sister that tells you that she took pictures at her company Christmas party and thought you might like to see them. You think to yourself that it's nice of her to send them as you open the attached "photos." The attachment doesn't open. Oh well, it wasn't that important anyway. You delete the e-mail and go on with your work. What you don't know is that 100 of your closest friends just got an e-mail from you asking them to look at your party photos.
E-mail viruses use social engineering to get you to run them on your computer. They look like they came from some one you know and have text such as the party photo example above. An infected computer can send out many thousands of copies of the virus. Some viruses only send copies of themselves, but other will actually take a random file from your hard disk and send it out as well. This poses a huge security risk for business and home users, as well as the potential for some very embarrassing situations.
These viruses get their e-mail addresses from the user's address book, inbox, hard disk or even the Internet. They often will put another address they find in the return address field. This prevents you from even knowing whose computer sent you the virus. The only way to stop it would be to track down the third party who is infected, but that is nearly impossible.
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The fact that viruses use random return addresses means that you shouldn't go attack the person whose e-mail address it appears to have come from. They are probably just a victim like you. Just delete the messages without opening them.
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