“My computer is acting up. It must be a virus.” You’ve undoubtedly heard comments like this or even thought this yourself. In actuality, most computer glitches are caused by software conflicts or user error.

Viruses aren’t as common as other computer problems. They’re found in about 0.15 percent of emails, according to the latest figures from MessageLabs (www.messagelabs.com), a provider of Internet security products that each day analyzes more than 180 million emails worldwide for its business clients.

That makes viruses less prevalent than phishing attacks that try to trick you into revealing your credit card, banking, or other personal information, which make up about 0.45 percent of emails. The most common email problem, however, is spam, those unsolicited, untargeted commercial messages sent in bulk, with such messages comprising a whopping 44.96 percent of all emails.

But viruses do get a lot of attention, and it’s easy to see why. They have an ominous and mysterious aura. And they can do serious damage, including wiping out all the data on your hard drive. Some may not do overt harm but instead scare you with a pop-up text message such as “Gotcha,” a photo of a raised middle finger, or a sinister audio or video file.

Computer viruses are simply small computer programs. Like human viruses, computer viruses can replicate, spreading like a disease from one computer to another through email or, less commonly, through infected CD-ROM discs, USB drives, music and other file-sharing networks, and Web sites.

All indications are that viruses are typically written by pranksters in their teens and twenties, according to virus experts. Some are written by truly disturbed individuals, the kind of sociopaths who indiscriminately slash tires. Some may be written in a more formalized way, by members of organized crime families or foreign terrorist groups. And some are intended to be “good viruses” that delete other viruses, but they may inadvertently cause harm. For instance, they may delete a vital system file by mistake.

Viruses may be written from scratch by programmers. Or they may be created with virus-writing kits, requiring no programming knowledge. Some virus writers write viruses for the intellectual challenge, never intending to release them. Some of these viruses get released accidentally. Web sites and online chat rooms exist where virus writers ask questions, trade tricks, and boast of their exploits.

The first line of defense against viruses, as with every potential computer disaster, is to make regular backups of the vital data stored on your hard drive. Ideally you should periodically do this to a medium that’s not continuously connected and accessible, to prevent a virus from infecting it too.

The next safety step is to use antivirus software. Top programs include Symantec’s Norton AntiVirus, available separately or as part of other Symantec products (www.symantec.com), and McAfee VirusScan (www.mcafee.com), also available separately or as part of a larger suite of other products.

Antivirus programs scan relevant files looking for the specific programming code, or signatures, of known viruses. They also look for common behaviors of viruses. To avoid conflicts, you should use only one antivirus program at a time.

Another excellent program, which can be used in conjunction with antivirus and other security programs, is Spybot Search & Destroy (www.safer-networking.org). It removes spyware and other malware and is a superb example of international altruistic entrepreneurship.

The program is written and supported by German software engineer Patrick Kolla and the volunteers who work with him, and it’s distributed by Kolla’s Irish company Safer Networking Ltd. The program, which has won many awards for its effectiveness, is free for noncommercial use, supported by donations. Fees for corporate use depend on the size of your network.

Also, in addition to protecting yourself with the above software shields, you should be careful about email attachments. Don’t open any from people you don’t know. If you receive an attachment from someone you do know but weren’t expecting it, it can be good practice to contact the sender to verify that the person actually intended to send it.

It’s also important to keep your operating system up-to-date, ideally directing it to download bug fixes and other updates automatically. It’s equally important to keep your antivirus and other security software up to date by doing the same.

Users of Microsoft Windows and Windows programs are most vulnerable to viruses, in part because of their market share and in part because there’s a hostility in the virus underground toward big business that Microsoft represents. But Apple Macintosh and Linux users also need to be careful.