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Virus Basics

This information gives an introduction to viruses and ways to avoid them.

Frequently Asked Questions

For More Information

US-CERT offers many resources to help you create a more secure home computing environment. These documents may be of particular interest if you have concerns about viruses and Trojan horses:



Frequently Asked Questions

What is a virus?
A computer virus is a program that spreads by first infecting files or the system areas of a computer or network router's hard drive and then making copies of itself. Some viruses are harmless, others may damage data files, and some may destroy files. Viruses used to be spread when people shared floppy disks and other portable media, now viruses are primarily spread through email messages.

Unlike worms, viruses often require some sort of user action (e.g., opening an email attachment or visiting a malicious web page) to spread.

What do viruses do?
A virus is simply a computer program--it can do anything that any other program you run on your computer can do. Some viruses are designed to deliberately damage files, and others may just spread to other computers.

What is a worm?
A worm is a type of virus that can spread without human interaction. Worms often spread from computer to computer and take up valuable memory and network bandwidth, which can cause a computer to stop responding. Worms can also allow attackers to gain access to your computer remotely.

What is a Trojan horse?
A Trojan horse is a computer program that is hiding a virus or other potentially damaging program. A Trojan horse can be a program that purports to do one action when, in fact, it is performing a malicious action on your computer. Trojan horses can be included in software that you download for free or as attachments in email messages.

Can I get a virus by reading my email messages?
Most viruses, Trojan horses, and worms are activated when you open an attachment or click a link contained in an email message. If your email client allows scripting, then it is possible to get a virus by simply opening a message. It's best to limit what HTML is available in your email messages. The safest way to view email messages is in plain text.

How can I avoid a virus infection from email?
Most users get viruses from opening and running unknown email attachments. Never open anything that is attached to an email message unless you know the contents of the file. If you receive an attachment from a familiar email address, but were not expecting anything, you should contact the sender before opening the attachment. If you receive a message with an attachment and you do not recognize the sender, you should delete the message.

Selecting the option to view your email messages in plain text, not HTML, will also help you to avoid a virus.

What are some tips to avoid viruses and lessen their impact?

  • Install anti-virus software from a reputable vendor. Update it and use it regularly.
  • In addition to scanning for viruses on a regular basis, install an "on access" scanner (included in most anti-virus software packages) and configure it to start each time you start up your computer. This will protect your system by checking for viruses each time you run an executable file.
  • Use a virus scan before you open any new programs or files that may contain executable code. This includes packaged software that you buy from the store as well as any program you might download from the Internet.
  • If you are a member of an online community or chat room, be very careful about accepting files or clicking links that you find or that people send you within the community.
  • Make sure you back up your data (documents, bookmark files, important email messages, etc.) on disc so that in the event of a virus infection, you do not lose valuable work.
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