The discs, known as CD-Roms (compact disc read only memories), are an increasingly popular format for those who need to carry or archive large amounts of information.
One CD-Rom, resembling the more well-known shiny music platters, can replace an entire encyclopaedia, or carry detailed graphical and textual information on every painting in an art gallery's collection. Often the discs cost several thousand pounds.
But the data on a CD-Rom is designed only to be read, not altered. So a CD-Rom with a virus on it cannot be cleaned up in the same way as programs held on floppy disk. Richard Ford, editor of Virus Bulletin, said yesterday: 'The only use for an infected CD-Rom is as a frisbee.'
The problem is that a virus on a CD-Rom can spread to the computer system that reads it. Transmission can occur via small functional programs, such as routines that will speed up access to the information, included on the disc in addition to the bulk data itself.
During December, virus specialists heard of four separate reports of infected CD-Roms. They fear this is the start of a growing trend and are warning computer users to scan new CD-Roms for viruses just as they would ordinary software arriving on a floppy disk. Scanning will be a time-consuming process since CD-Roms hold huge amounts of data in hundreds of compressed files.
Computer viruses can cause relatively mild effects, such as messages flashed up on screen, or potential disasters if the rogue code disrupts or erases valuable data. The cases reported last month occured on discs carrying so-called 'shareware', software that people try out before handing over a fee to its author.
Among computer hobbyists, shareware is a popular way of testing new computer programs. The difficulty is that shareware is often second-hand, copied and collated from electronic message centres, called bulletin-boards, which are renowned sources of virus infection. Mr Ford said: 'The larger reference discs produced by reputable companies are fairly reliable but it doesn't really matter where a CD-Rom comes from; it should always be checked.' He added: 'Shareware discs are very risky indeed. I would advise people not to use these CD-Roms on machines that hold critical data.'
The reported infections include two shareware collections. The first is called Software Vault collection 2, published by the American Databank Corp, which was infected with a virus activated every day between 9am and 10am. This requires the user to enter the answer to a simple mathematical problem before they can use their computer.
The second is called Night Owl 10, infected with a relatively harmless virus called 'Lapse'. The manufacturers of both discs admit the infections and are expected to withdraw the CD-Roms.Reuse content