The Virus Threat

Lesson 2: Virus Catagories

There are many different types of viruses, knowing a little about the different types can help you defend against viruses.

Symantec, one of the top Virus protection program producers, lists five categories of viruses:

File infector viruses.
Boot sector viruses
Master boot record viruses
Multi-partite viruses
Macro viruses

They do not list Trojan horses or worms as viruses however. They list these as separate entities. For the purposes of this course we shall consider both as viruses. A Trojan horse is a virus that appears as though it is a desirable program, however it contains malicious code. A worm is a program that self replicates itself and sends itself from computer to computer, a worm is not considered a virus because it is the whole program that is self replicating rather than infecting files with the virus code.

File Infector

A file infector virus is exactly what it sounds like it is a virus that infects files. When you run a program infected with a file infector virus the virus copies the infection code to other executable programs on your computer. Files that are most commonly infected by this type of virus are .exe or .com files, although any file that can be executed can be infected.

An example of a File Infector virus is the Cascade virus. The Cascade virus is all but extinct now, but it is a good example of a file infector virus. The original version of Cascade was programmed to deliver its payload between October 1^st and December 31^st 1988. When the virus "goes off" the characters on the screen begin to "fall" ending up in a pile on the bottom of the screen. There are a number of variants, and every now and then a new variant appears, but they are all basically the same.