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Viruses, Trojans and Worms

Trojans, Viruses & Worms

The commonest computer threats are viruses.


A virus is a computer program which changes the way in which the computer operates without the knowledge of the user.

A virus must execute itself and it must replicate itself, or make additional copies of itself. Some viruses damage computers by altering programs, deleting files or other malicious activities. Others simply announce their presence by audio, video or text messages. Even viruses which don't do any real damage are a nuisance as they waste memory and can slow down a computer. Most viruses can cause a system to behave oddly and they may contain bugs (programming errors) which can cause the system to crash.

Viruses can spread using many methods including attaching themselves to other files such as programs and documents. Viruses are given different names depending on the way they spread. Some of these different types of virus are explained in later sections. Simply spreading from computer to computer is not itself a major problem. Most viruses contain a payload, a program that they will execute in addition to spreading themselves. The effects of common virus payloads include deleting or corrupting files or disks and passing on usernames and passwords.

Next: Types of Virus