The 1980s saw the advent of widespread (and potentially damaging) computer virus infection of both personal computer and mainframe systems. The computer security field has been comparatively slow to react to this emerging situation. It is only over the last two years that a significant body of knowledge on the operation, likely evolution and prevention of computer viruses has developed. "A Pathology of Computer Viruses" gives a detailed overview of the history of the computer virus and a technical review of the principles of computer virus and worm operation under DOS, Mac, UNIX and DEC operating systems. The author considers the possible extension of the threat to the mainframe systems environment and suggests how the threat can be effectively combatted using an antiviral management plan. The author addresses the latest developments in "stealth" virus operations, specifically the trend for virus authors to adopt extensive camouflage and concealment techniques, which allow viruses to evade both existing antiviral software and to avoid detection by direct observation of machine behaviour.