More dangerous versions of the virus have also begun to crop up, with lethal consequences, experts say.
Concept was first discovered last August. Its rapid spread is thought to be due to firms' increasing use of internal electronic mail to streamline their operations and cut down on paper.
"At least 25 per cent of corporations either have or have had infections by Concept, because it infects Word documents," says Alan Solomon, chairman of S+S International. His company specialises in writing anti-virus software, and last year saw revenues grow by 80 per cent to pounds 14m.
Industry analysts reckon that the first firm to be infected by Concept was Microsoft itself - the epitome of the modern corporation, where electronic mail is the main channel of communication. Last summer a CD-Rom sent out by Microsoft to software developers around the world contained documents infected with the virus, necessitating the hasty development of a "cure" for the problem by the company. However, the virus spread rapidly.
Concept - also known as Prank - is different from previous viruses because it can affect any computer that is running Version 6.0 or later of the Word word-processing program.
The virus conceals itself in a "macro" attached to a Word document. A macro is a mini-program that the computer's user creates to automate a common function, such as putting the date or address at the top of a page.
In newer versions of Word, the macro can be attached to a document, which can then be sent electronically to other people.
When a user opens an "infected" document, the virus macro copies itself into the standard template of the user's machine, and infects all subsequent documents written by the user.
"With the increasing amount of e-mail being used, it's easy to get infected," says Mr Solomon. "Someone sends you mail with a document on it, you open it, and bang - you're infected."
Although the Concept virus is harmless, there are now five known "macro viruses", and experts reckon that the newest ones are becoming increasingly dangerous. The latest, called Hot, spreads when an infected document is loaded using Word. It then sets a "hot date" 14 days ahead, when it begins to erase documents on the user's computer at random.Reuse content