A virus is a computer program that can attach itself to host files and replicate itself repeatedly, usually without user knowledge or permission. Viruses attach to files in such a way that when the infected file executes, the virus also executes. Other viruses can sit in a computer's memory and infect files as the computer opens, modifies, or creates new files.

A computer system infected with a virus can display various symptoms. Some viruses damage files and operating systems, but neither symptoms nor damage are definite indicators to the presence of a virus; a virus that is not overtly destructive is still a virus.

Currently there are computer viruses written for nearly all operating systems including DOS, Windows, Mac OS, and Unix.

Computer viruses, similar to human viruses, usually adhere to three main criteria:

• Self-Replication: Computer viruses will copy themselves onto any host program or file that they are programmed to infect, just like a human virus will travel from one person to another. Commonly, viruses replicate themselves in the form of e-mail attachments that are sent to addresses harvested from an infected computer's address book.
• Host File or Program: Similar to a human virus, which can only survive if it infects a living person, a computer virus can only exist if it has infected a host file.
• User Activation: A computer virus will not activate and spread until a user executes it. Consequently, virus authors often use social engineering to trick victims into activating a malicious virus. A recent example involved an e-mail attachment which purported to show pictures of the capture of Osama Bin Laden. When users clicked the attachment, they were infected with a malicious virus.

Virus hoaxes are either deliberate or unintentional e-mail messages warning people about a phony virus or other malicious software program. Some hoaxes create as much trouble as viruses by causing massive amounts of unnecessary e-mail.

Virus Detection and Prevention Tips

Get protected.
If you don't already have virus protection software on your machine, you should. If you're a home or individual user, you should install the latest anti-virus software for your personal computer. If you're on a network, check with your network administrator first.

Scan your system regularly.
If you're loading anti-virus software for the first time, it's a good idea to let it scan your entire system. Often the anti-virus program can be set to scan each time the computer is rebooted or on a periodic schedule. Some will scan in the background ("real time") while you are connected to the Internet. Make it a regular habit to scan for viruses.

Don't open attachments.
One of the best ways to prevent virus infections is not to open attachments, especially when dangerous viruses are being actively circulated. In fact, e-mail attachments are the number one attack vector for infection from viruses. Even if the e-mail is from a known source, be careful. Many viruses take the mailing lists from an infected computer and send out new messages with its destructive payload attached. Always scan the unexpected attachments for viruses.

Stay informed.
There are new virus and security alerts almost every day. Keep up-to-date on breaking viruses and solutions. Remember that the bad guys are going to try to use social engineering to exploit you. Staying informed is the perfect countermeasure against that.

Update your anti-virus software.
Now that you have virus protection software installed, make sure it's up to date. Most anti-virus programs have a feature that will automatically link to the Internet and add new virus detection definitions whenever the software vendor discovers a new threat.

Information on how to install and use anti-virus programs.