4.2- The Missing BrainThe Brain virus was first observed in October, 1987, making it one of the first DOS viruses seen in the world [15]. It infects diskette boot sectors, and becomes active in a system when that system is booted from an infected diskette. Unlike most boot viruses today, Brain does not infect boot sectors of hard disks. In the early days of PCs, most PCs were booted from diskettes and did not have hard disks. This provided a perfect medium for Brain to spread. Diskettes used in an infected system became infected themselves, and could carry that infection to other systems. Brain spread around the world in just this way. Beginning with the introduction of the IBM PC-XT in 1982, the PC industry made a transition to systems that have hard disks. Unlike their predecessors, these systems were not booted from diskettes as frequently. When they were booted from diskettes, it was typically for some special activity, such as system maintenance. Once that activity was concluded, the system was rebooted from the hard disk. It became very uncommon for a system to be booted from a diskette and then used for an extended period of time, with more diskettes being inserted into the system. This denied the Brain virus the opportunity to spread in most cases. The world became a much more difficult place for the Brain virus to spread, and its prevalence declined. This decline in prevalence occurred before we started gathering accurate statistics about virus incidents, so we cannot illustrate it quantitatively. Anecdotal evidence and our own informal statistics from the late 1980's, however, suggest that the Brain virus was substantially more common than it is today. While the Brain virus is still seen on rare occasions, it does not spread well today. We sighted the Brain virus several times from mid-1988 until mid-1990, but since 1990 it has only appeared in our sample population once, in early 1992.
|