These ten account for over two thirds of all incidents. The one hundred other viruses that have been seen in incidents in the past year account for less than a third of the incidents. The top ten viruses account for two thirds of all incidents. All of them are boot-sector infectors. Curiously, the ten most prevalent viruses are all boot viruses.
Boot viruses infect boot sectors of diskettes and hard disks.
When a system is booted from an infected diskette, its hard disk becomes infected. Boot viruses have not always been dominant. Three years ago, the second and third most prevalent viruses were file infectors, as were 4 of the top 10. The total incident rates for boot infectors and file infectors were roughly equal. Figure 5 provides another view of what has happened to the relative prevalence of these two types of viruses over time.