Almost every day we are warned about a virus making the rounds. Although it may seem that they spread independently from computer to computer, in fact they do not… they need people to spread, just like a cold. Viruses are like sneezes, except that the sneezes are emails. Innocently attached to an e-mail message, a virus does its damage by delivering its payload – a malicious piece of code – once it has reached the destination computer.

Here’s a quick primer on the different types of viruses, how they typically propagate, and what kind of damage they can do.

Simple Virus

This type of virus works by attaching itself to a program file, and is activated when you launch that program. The virus then takes control of your computer, attaches itself to another program file, and the cycle continues.

Encrypted Virus

Every virus is a piece of code, or a specific sequence of bytes. This sequence is known as a signature, and in an encrypted virus, this signature is scrambled, making it difficult for virus scanning software to detect.

Polymorphic Virus

As with the encrypted virus, creators of polymorphic viruses encrypt the virus’ signature. What makes this type of virus so difficult to detect, however, is that the decryption routine used to unscramble the encryption is also encrypted.

Macro Virus

Almost 75% of viruses reported today are macro viruses. They are easy to write and generally spread via programs that make use of macros, such as MS Word.

Trojan Horse

Like the legendary Trojan Horse of Greek mythology, this type of virus is actually a nasty program disguised as a benign program, like a screensaver. It does its damage when it is executed, typically stealing passwords or spreading viruses.

Worms

Unlike a true virus, which needs users (and usually their e-mail) to spread from computer to computer, a worm is self-propagating. Because of this, a worm can spread to hundreds of thousands of computers in an incredibly short time, doing substantial damage as it does. Not only can a worm attach itself to every email address in your MS Outlook address book, but it can also stick to certain types of files on your computer which, when opened, will start the whole propagation process again.

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