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or when certain other conditions are fulfilled. Most viruses are resident.
The second category is System or Boot-Record Infectors: those viruses which infect
executable code found in certain system areas on a disk which are not ordinary files.
On DOS systems, there are ordinary boot-sector viruses, which infect only DOS boot-
sector, and MBR viruses which infect the Master Boot Record on fixed disks and the
DOS boot sector on diskettes. Examples include Brain, Stoned, Empire, Azuza and
Michelangelo. Such viruses are always resident viruses.
But a few viruses are able to infect both (the Tequila virus is one example). These are
often called Multi-Partite viruses, though there has been criticism of this name;
another name is Boot-and-File virus.
File System or Cluster viruses (e.g. DIR II) are those which modify directory table
entries so that the virus is loaded and executed before the desired program is. Note
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