2. Viruses and worms

Certain types of malware create problems by copying themselves.

Virus

This is a piece of code that is designed to replicate itself throughout the computer or network. For example it will add itself to a file on your computer and, every time the file is opened, the virus runs and once more seeks out other files to infect. Viruses can be difficult to get rid of since, if a single infected file is left, it will eventually re-infect many other files on your system.

A virus will often be designed to delete or corrupt data.

New viruses appear every day, and so the anti-virus companies are constantly updating their software to keep up with them.

Worm

This behaves just like a virus, but does not necessarily harm files. Instead, it tries to copy itself so many times that the whole system runs out of resources and crashes.

One of the most famous worms is the ILOVEYOU email worm. In 2000, it managed to infect nearly 10% of all internet connected computers, causing an estimated $8 billion in damages in terms of wasted time dealing with it.

Trojan

Named after the famous Greek story of the Trojan Horse.

A trojan appears to be a harmless application - a downloaded game for example. You play the game, but meanwhile it is running in the background carrying out its real intent - maybe letting someone else have remote access to your computer.