Virus: A self-replicating part of computer code that can partially or fully connect itself to files or applications, and can cause your computer to do something you don't desire it to do.

Computer viruses are the "common cold" of current technology. They can spread quickly across open networks such as the Internet, causing billions of dollars value of damage in a short quantity of time. Five years ago, the possibility you'd receive a virus more than a 12-month period was about 1 in 1000; today, your probability have dropped to about 1 in 10. The vital information:

• Viruses come in your system via e-mail, downloads, infected floppy disks, or (occasionally) hacking.
• By definition, a virus have to be able to self-replicate (make copies of itself) to spread.
• Thousands of viruses live, but few are establish "in the wild" (roaming, unchecked, across networks) because most known viruses are laboratory-made, not at all released variations of common "wild" viruses.
• Virus behavior can range from annoying to unhelpful, but even relatively benign viruses tend to be critical due to bugs introduced by sloppy programming.
• Antivirus software can notice nearly all types of known viruses, but it must be efficient regularly to maintain effectiveness.

It is just a computer program. Like any other program, it contains commands that tell your computer what to do. But different an application, a virus typically tells your computer to do something you don't want it to do, and it can typically spread itself to other files on your computer--and other people's computers.

If you're lucky, a virus will perform only a benign "personality quirk," such as causing your computer to make apparently random bleeps. But a virus can be very unhelpful; it might format your hard drive, overwrite your firm drive boot sector, or delete files and make your machine inoperable.