Watch out for worms and polymorphs

Machine nasties you should try to avoid at all costs

Atom: A Microsoft Word macro virus, it infects any document opened or saved. When opening an infected document on December 13 of any year, it will delete all files in the current directory.

AutoStart 9805: A worm that exploits the autoload setting on the QuickTime control panel of a Macintosh. Destroys data. Has spawned a number of dangerous variants.

Bandung: A Word macro virus that deletes all files on the computer's hard drive.

Brain: Resident, stealth infector of floppy boot drives, this was discovered in 1986 and is the oldest known PC virus. Although there are several variants, the original displayed a message starting off with "Welcome to the dungeon".

CAP: This Word macro virus is now one of the world's most common. An irritant rather than a serious problem, it infects the program's document template and overrides menu items.

Cascade: Resident of .com PC files, it gained widespread attention towards the end of 1988 by causing the letters to fall to the bottom of the screen. New variants reformat the hard disk.

Concept: This was the very first Word macro virus, and is known to alter menu items.

Dark Avenger: A resident virus written in Bulgaria that can cause serious damage. Writesrubbish randomly to disk. Displays the message "Eddie lives somewhere in time".

Invader: It plays music around 30 minutes after infection, and will overwrite part of the hard disk.

Maltese Amoeba: A destructive polymorphic virus that overwrites part of the start-up disk on November 1 and March 15. Its long dormant period highlighted the shortcomings of virus scanner software. Displays pieces of Blake poetry on infection.

Michelangelo: Destructive resident. Writes rubbish to start of disk details. Destroys all data on infected files on March 6.

Pathogen: Originated in the UK. Alters the file date, and writes rubbish to disk at 5pm on Mondays. Displays the message "Smoke me akipper, I'll be back for breakfast".Unfortunately, some of your data won't.

Stoned: Resident of DOS sector, it causes disk corruption. One of the most common viruses, it damages floppies and displays the message "Your PC is now stoned". Sparked a flurry of variants.

Tremor: Destructive polymorphic virus, it makes the characters on-screen appear to shake. Waits three months after infection before activating.

4096: Causes irreparable damage to PC disk files. A "stealth" virus, it works very slowly. Programmes and files are infected when executed or read.

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