Computer viruses infect executables files. These include Macintosh system files (such as system extensions, INITs, and control panels) and applications (such as word processing programs and spreadsheet programs). A virus is inactive until you execute an infected application or start your computer from a disk that has infected system files. Once a virus is active, it loads into your computer's memory and, at the least, attaches copies of itself to applications or system files on disks you access.Some viruses are programmed specifically to damage the data on your computer by corrupting programs, deleting files, or even erasing your entire hard disk. Many of the currently known Macintosh viruses, however, are not designed to do any damage. They simply replicate themselves and may display messages. However, because of bugs within the virus, your system may behave erratically or crash unexpectedly.Even if you've never had a virus on your computer, you've probably heard of some of them. For example, the WDEF virus causes your computer to beep, frequently crash, or display fonts incorrectly. The nVIR virus beeps every 8 to 16 times you start your computer. One strain of the nVIR will generate the phrase "Don't Panic" from your computer's speaker instead of beeping. Its designer probably found the idea amusing, but most victims do not.Computer viruses don't infect files on write-protected disks, and they usually don't infect documents. They don't infect compressed files either. However, applications within a compressed file could have been infected before they were compressed. Viruses also don't infect computer hardware, such as monitors or computer chips; they only infect software. In addition, Macintosh viruses don't infect DOS-based computer software and vice versa. For example, the infamous Michelangelo virus does not infect Macintosh applications. Finally, viruses don't necessarily let you know that they are there, even after they do something destructive. MBDF virusesFirst appearing in 1992, MBDF-A originated from a Trojan Horse named Tetracycle. Additionally, MBDF-A was found to be distributed in versions of Obnoxious Tetris and Ten Tile Puzzle. MBDF-B is a related strain discovered in November of 1993. The MBDF-A and B viruses infect applications as well as system files and spread rapidly. MBDF-A and B cause occasional crashes, particularly if commands are selected from menus when running System 7.0.1.The MBDF-B virus spreads on all types of Macs except MacPlus systems and (perhaps) SE systems; it may be present on MacPlus and SE systems and not spread, however.The virus does not necessarily exhibit any symptoms on infected systems. Some abnormal behavior has been reported in machines infected with MBDF-A, involving system crashes and malfunctions in various programs, which may possibly be traced to the virus. Some specific symptoms include:* Infected Claris applications will indicate that they have been altered * The "BeHierarchic" shareware program ceases to work correctly.* Some programs will crash if something in the menu bar is selected with the mouse.The MBDF-B virus should behave similarly and will spread under both System 6 and System 7.