Viruses, Trojan horses, email worms, and other nasties are unfortunately an everyday fact of life on the internet. In this lesson, you'll learn how each type of threat attacks your computer and what you can do to prevent an attack.

Identifying the Types of Threats

Part of the reason computer viruses are so nasty and pervasive is that the people who write them are creative -- always coming up with new ways to cause problems. Some of the common virus types on the loose today include:

• File virus: A virus that infects a program file, and every time you run that program, the virus puts itself in memory, and infects all other program files that you run afterwards. It may also do something harmful to your computer.
• Boot sector virus: A virus that hides in the startup area of a disk and loads itself into memory when you start the computer using that disk. It's then able to infect every writeable disk (such as a floppy) that you insert into the computer.
• Trojan horse: A program that appears to do something useful but actually delivers a harmful effect such as opening a security hole, spreading itself via email, or deleting or damaging files. Trojan horses can also attach themselves to web pages, such that when you view the web page, you get infected.
• Worm: A program that spreads by making copies of itself. It may or may not do anything additional.
• @m: A mailer is a type of worm that attaches itself to email that you send, but does not send itself out automatically.
• @mm: A mass mailer is a type of worm that automatically sends itself to multiple addresses from your address book. It makes up a bogus message.
• Backdoor: A program that sends information back to its creator about the infected system, making it easy for that person to hack into the infected system and take control of it or read sensitive data.
• Blended threat: A combination of infection types in a single item. For example, a worm that also infects a boot sector, deletes important files, or opens a security backdoor would be a blended threat, as would a backdoor program that distributes itself as a mass mailer.