Understanding Computer Viruses and How They Spread

Practically everyone with a computer has heard of computer viruses, but not everyone knows exactly what they are or how viruses can affect their computer. A virus is a small, self-replicating program that invades your system through an email attachment, download, or website. People create viruses for various reasons, ranging from mild pranks to malicious destruction.

As a result, the effects of a virus can vary from moderately annoying to absolutely crippling. The Wazzu virus, for example, attached itself to Microsoft Word documents and inserted the nonsense word "?wazzu?" in the document and rearranges random strings of words. A virus such as this one was definitely annoying, but not necessarily catastrophic.

Some viruses can be much more serious, however, threatening to destroy data or even erase your entire hard drive. One such virus spread by email, Worm.ExploreZip, takes control of the infected computer's email system and automatically responds to all incoming mail messages. It also sends destructive email attachments that can erase certain types of files.

Viruses can be harmful, but it's also important to understand the role you play in propagating them. Viruses generally do not invade a computer without user action. In other words, you set the virus loose on your own computer by opening infected programs or files. If you take certain precautions, such as not opening email attachments from unknown sources and running antivirus software, you can minimize the risk of infecting your own computer.

Types of computer viruses

By familiarizing yourself with the types of viruses that exist, you'll be better able to prevent them.

Program viruses attach themselves to executable files such as .exe files. Each time you run the program, the virus duplicates itself and attaches to other programs. Sharing programs with other computers, either by disk or on a network, spreads these types of viruses. You are also at risk for this type of virus when you download applications from the internet.

Macro viruses are contained in the macro commands for programs such as Microsoft Word and Excel. Perhaps the most prevalent type of virus, they attach themselves to files (as opposed to programs) and are some of the hardest viruses to detect.

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Boot sector viruses infect particular files on your hard drive that execute when you boot your computer. When you boot from the infected disk, the boot sector virus attaches itself to the boot files on your hard drive.

Trojan horses aren't viruses by definition because they don't replicate themselves. They do, however, possess the same potential to damage your hard drive if they go undetected. They typically arrive in an email with a vague subject line or a promise of entertainment. When let loose, the Trojan horse—much like its namesake in Greek mythology—introduces a slew of unwanted attackers that can wreak havoc on your hard drive.

How are macro viruses different from other computer viruses?

The basic differences between macro viruses and other types of viruses are how they are created and how they are spread. A regular virus is written expressly to reproduce and spread from computer to computer. Most target the boot sector of your computer and activate when you start your system. Macro viruses, on the other hand, are often created by user error and are spread via the macro features of programs like Microsoft Word and Excel.

A macro is a series of commands grouped together as a single command in order to expedite time-consuming procedures. In the process of creating these shortcuts, users sometimes unknowingly record harmful macros in their documents and pass them on to other users. If another user opens the file with the macro feature enabled, the macro virus may infect their computer.

There are thousands of documented macro viruses, and more appear every day. Most are written specifically for Microsoft Word and Excel, so if you don’t use those programs, you’re in little danger of being infected by a macro virus. However, don’t let the threat of viruses deter your use of these programs; the macro features of Word and Excel are extremely powerful.

Just be on your guard: Your best defense against macro viruses is to install antivirus software and to keep it updated with the latest virus information.

How do computer viruses spread?

Computer viruses usually spread in one of three ways: from removable media; from downloads off the internet; and from email attachments.

Although the internet gets a bad rap as a source of viruses, you're no more likely to contract a virus from the Web than you are from packaged software. Still, scan everything you download, and update your antivirus software regularly.

E-mail is not the virus breeding ground it's made out to be, either. In fact, it's nearly impossible for a virus to be transmitted by plain-text email. Most viruses can only spread via attachments, either rich-text email or attached applications. Using antivirus software, scan attachments from people you know, and never open attachments from people you don't. If you're a Microsoft Outlook user, you can also select security preferences that keep email-borne viruses from exploiting the close relationship between Outlook and the Windows operating system.

These precautions will minimize the risk of infecting your computer as well as keep you from spreading viruses onto others.

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