For most users, computer virus is synonym of the worst nightmares that can happen on their system. Yet some famous researchers keep insisting that it is possible to use the replication mechanism of the viral programs for some useful and beneficial purposes. This mechanism does not specify explicitly that it is malicious. Intuitively, computer viruses are just a kind of technology. As with any other kind of technology, they are ethically neutral – they are neither bad nor good – it is the purposes that people use them for can be bad or good. So far they have been used mostly for bad purposes. It is therefore natural to ask the question whether it is possible to use this kind of technology for good purposes. Indeed, Fred Cohen (Cohn 1994, p.15), one of the most active proponents of the idea has attempted to implement the idea.

The File Compressor Virus is one of the oldest ideas for good viruses of Cohn. The idea consists of creating a self-replicating program, which will compress the files it inflects, before attaching itself to them. Such a program is easy to implement and it has already been done. There is a family of MS-DOS viruses which appends itself to the executable files, then compress the infected file, and then deposits a small decompressor that would decompress the file in memory at runtime.

Some people have had the idea to develop an “anti virus” virus – a virus that would be able to locate other malicious ones and remove them, Such a self-replicating anti-virus program would have the benefits to spread very fast and update itself automatically. Several viruses have been constructed along these ideas. Some of them locate a few known viruses and removed them from the infected files, others attach themselves to the clean files and issue an error message if another piece of code becomes attached after the unwanted virus.

It appears that all these good viruses are far from successful as they have too many undesirable or even destructive properties so that the existence of good virus is highly questionable.