The Concept Nuclear virus is coded in a different manner from past strains and cannot be detected by standard scanners. A benign version - dubbed a "prank macro" - was discovered in Britain several weeks ago, while outbreaks have been reported in the U.S., Canada and Europe. Several days ago an unnamed London-based medical charity unwittingly downloaded a malignant version of the virus when a member of its staff accessed a site about French nuclear tests on the Internet. The charity, aware that a dangerous virus was about, scrutinized the file and spotted the virus. Still assuming it to be harmless, staff members showed it to Relflex Magnetics, an anti-virus company. The file was tested on a Pentium PC and wiped clean the system's hard disk. "This is just the tip of the iceberg," said Andy Campbell of Relflex Magnetics. "I have already seen a similar virus which attacks spreadsheet files. A lot of people are going to get burned by these macros." Concept Nuclear is written in a "macro" language, a mini-programming language contained within a word-processing or spreadsheet package that allows users to execute long strings of commands by pressing some key. Other viruses, by contrast, are made up of pieces of an operating system's code. Concept Nuclear will not wipe out the contents of a hard disk until a prearranged date - April 5 in the medical charity's case. A spokesman for Microsoft said the company hoped to develop a technique to scan for all destructive macros. But many experts fear this will prove extremely difficult. "You can check current viruses easily by scanning for them on a computer's operating system," an expert said. But to check for the new viruses, you will have to go through every single document on a hard disk and there could be thousands. This is very scary indeed."