About 1.3 billion people use one or other version of Microsoft's Windows operating systems, and well over a billion have downloaded Mozilla's Firefox web browser. Minor variations aside, every copy of these products--like all other mass-market software--has exactly the same bits in it.
This makes such software a honeypot for hackers, who can write attack code that will cause precisely the same damage to, say, every copy of Windows 7 it infects. Worse, the bad guys can hone their attacks by practising on their own machines, confident that what they see will be what their victims get.
This computing monoculture--which also extends to the widespread use of particular pieces of hardware, such as microprocessors from Intel and ARM--has long been the bane of technologists. In the face of a near constant onslaught from hackers, antivirus software is frequently several steps behind the foe.
Symantec, one of the commercial pioneers of online security, estimates that antivirus software now stops only 45% of attacks. The firm recently declared that this approach was "dead" and a new one was needed.