What follows is a list of the most common viruses. In the press these are often referred to as "computer viruses", but there are many sub categories. Many of these can appear one in combination with another, for example as in the I LOVE YOU-virus where a "worm" hid itself behind a "Trojan horse".

Bootvirus

Bootvirus is the oldest form of virus. They hide in the boot sector of discs and hard discs. When the computer boots (restarts) the virus is activated ad the proceeds to contaminate all discs. You can get the contagion by attempting to boot from a contaminated disc.

File Virus

File viruses place themselves in program files and are activated when the host program is executed. When activated the virus spreads to other .com and .exe files. It does not necessarily contaminate all files, and often uses logical tricks to avoid detection. You get them from contaminated files, such as files you download from the Net.

Macrovirus

They are written in macro language for programs such as Word and Excel, and can therefore be spread to most computer types. Most macros hide in Word documents (not in the text, but in the macro part) and are activated when you open the document, and then start spreading to other documents. In the later versions of Word there is a function to shut off macro, which can sometimes be useful.

Trojan Horses

A Trojan Horse is a small piece of software that accompanies other, innocent programs. A Trojan works in the background without the user noticing it. Famous Trojans include BackOrifice and NetBus but there are many more. NetBus for example allows the intruder to remote-control your computer - he can then listen thru your microphone, change your hard disk, etc.

Logic Bomb

A Logic Bomb is a piece of code that is activated when certain conditions apply. Say for example that a programmer gets fired, he then writes a code which erases all data exactly one year after said date.

Worms

Worms are programs which copy themselves, and they need no host file. A worm can be compared to a chain letter. If a worm has, and it does not have to have, a destructive mission it can destroy lots of data VERY fast. The most important distinguishing characteristic of a worm is the spread-rate. A virus can take hours or even dates to spread, while a worm can take as little as a few seconds.

Droppers

The above phenomena can be combined. An example of droppers are Trojans that at a certain time "drop off" a virus. If a worm where to infect files while at the same time spreading new processes around - then you would have a huge problem. No such worm is known today.

Start Sector Infectors

Every logic unit, both hard and floppy discs, have a start sector. This applies even to discs that you cannot use as start discs. The start sector contains info on how the particular disc is formatted, what data it contains, and a small start program (a boot-program, that reads DOS system files).

The start program can give you the famous "Non-System Disc or Disc Error" if the DOS files are not present. It is THIS program that can get infected by virus. You can get a start sector virus by leaving a contaminated disc in the drive and rebooting your computer. When the start program is run, the virus transmits itself to the hard disc and contaminates the host computer.

Melissa/I Love You

This particular virus attaches a Visual Basic-script to e-mails. If it is opened/activated it will automatically search for the Outlook Express Address Book and re-send itself to everybody in that list. This leads to a large number of individuals being contaminated and that e-mail systems can be overloaded.

Very Joke/ExploreZip

A virus that works like Melissa but can cause a lot more damage. The virus can affect PC-users on Windows 95, 98 or NT using the e-mail program Outlook. The infected computer sends out answers to all UnRead mail. That way, it looks like the user has, in effect, answered. Every mail will have an attachment called "zipped_files.exe". if the recipient opens this attachment all Word-, Excel and Power Point-files will be erased.

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