A message sent with mischievous intent

Like their biological counterparts, computer viruses this week again proved that they may be small and seemingly innocuous, but they can spread havoc in their wake once they've found an unsuspecting host.

The Melissa virus, which spreads itself by e-mail, first made itself known last weekend, with devastating effect on business and other large organisation computer systems in particular, shutting down networks for hours. While it doesn't actually damage computer data, the virus sneaks into the popular Microsoft e-mail program Outlook and then mails a copy of itself to the first 50 addresses listed in the address book that is part of Outlook, including large group mailing lists that are organised under a single address heading. The resultant sudden mass mailing can bring down a system.

Appropriately enough, given the sexually-transmitted disease metaphor which surrounds computer viruses, the Melissa virus arrives in a system by accompanying a Word document that lists sexually explicit websites.

The growth of the business and home computing market and the emergence of malevolent computer viruses paralleled the discovery of the AIDS virus in the early 1980s and the two have remained imaginatively linked ever since. As a result, the language used to discuss them is one of the aspects of computer viruses that has always made them seem eerily and sinisterly alive, rather than the small, self-replicating computer programs they actually are.

Unlike bugs, which are accidental glitches in computer programs that the program developers failed to eradicate and which can cause their own set of problems, viruses are written deliberately and with mischievous - and, some would argue, often evil - intent. The people who write them tend to be part of "hacker" (or, more correctly, "cracker") culture - highly adept computer users outside the mainstream, some of whom relish demonstrating their computing prowess through their ability to crack computer security systems or by creating viruses.

Viruses are costly to companies because it can take hours and even days to clean up the mess they leave behind and then, a complete computer system has to be disinfected. Add into the equation the peripheral costs of recreating data that has been totally lost, as well as lost productivity time while computer systems are down, and it's easy to see how the cost of damages from a single virus in a large company can spiral into millions of pounds. Most viruses - some 99 per cent of them - reside in computer memory and await an opportunity to reproduce by planting themselves into whatever type of computer file it is their speciality to infect. Often these are the kinds of files that people tend to copy and move elsewhere onto a new computer, allowing the virus to reproduce itself in a new host.

Initially, most viruses were transmitted by floppy disks when a computer user inadvertently copied an infected file from the computer's hard drive onto a disk and then loaded the disk into another computer. Viruses can also spread by riding on files when they are transmitted over a network to another machine on the system.

But the advent of the Internet has created the perfect environment for cultivating viruses - files of all sorts are easily broadcast from computer to computer and with millions of computers interlaced into the Net, viruses can enjoy endless replication possibilities.

The most common type of virus in the "wild" (found out in the real world) is the boot sector virus, or BSV. These are carried into a system through an infected file on a disk and do nothing until the next time the computer user restarts (or "boots") a computer with the infected disk still in the A (floppy disk) drive. The virus then loads itself into computer memory and from then on, every time a disk is put into the computer's A drive, the virus places itself onto the disk and can be spread to other machines in the same way.

Another common form of virus is the macro virus, which is carried in the macros, or tiny computer programs, used in Microsoft's Word word processing program but also in other programs such as Excel and Lotus Amipro. According to industry experts, macro viruses account for 50 per cent of calls to anti-virus software help desks.

There are a wide range of other viruses, such as multipartite viruses, which spread themselves to multiple types of files and parts of a computer, and overwriting viruses which overwrite files with themselves, thus eradicating the original program in the file.

The virus software industry characterises viruses by two other criteria - how quickly they do their damage, and the type of damage they do. Some work quickly, some slowly, and some by stealth, in areas of the computer where they are hard to find. Some are defined, none too cheerily, as "fun" viruses because they do no real damage and just make a nuisance of themselves, perhaps adding a line of text to documents or flashing a message on the computer desktop or making the individual computer keys beep.

Others systematically demolish computer files, resulting in loss of data. Entire networks can be crippled, and some of the most dangerous viruses mail back the system manager's password to a third party, which lets that person get into the system and do whatever damage they like.

Not surprisingly, the proliferation of computer viruses has led to the creation of a large and highly profitable computer virus industry which provides relatively inexpensive programs to computer users to protect against the computer virus scourge. These act as prophylactics, detecting and eradicating infected files before they spread into a system. They also detect and "clean" infected files and areas of computer.

The firms, such as Symantec and McAfee, run huge research laboratories which study and grapple with existing viruses and, when a new menace such as Melissa is discovered, rush to create an antidote - much like the pharmaceutical industry.