A virus is a piece of programming code inserted into other programming to cause some unexpected and,
for the victim, usually undesirable event. Viruses can be transmitted by downloading programming from
other sites or be present on a diskette. The source of the file you're downloading or of a diskette you've
received is often unaware of the virus. The virus lies dormant until circumstances cause its code to be
executed by the computer. Some viruses are playful in intent and effect ("Happy Birthday, Ludwig!") and
some can be quite harmful, erasing data or causing your hard disk to require reformatting.

Generally, there are three main classes of viruses:

File infectors. These viruses attach themselves to program files, usually selected .COM or .EXE files. Some can infect any
program for which execution is requested, including .SYS, .OVL, .PRG, and .MNU files. When the program is loaded, the
virus is loaded as well.

System or boot-record infectors. These viruses infect executable code found in certain system areas on a disk. They attach
to the DOS boot sector on diskettes or the Master Boot Record on hard disks. A typical scenario (familiar to the author) is to
receive a diskette from an innocent source that contains a boot disk virus. When your operating system is running, files on the
diskette can be read without triggering the boot disk virus. However, if you leave the diskette in the drive, and then turn the
computer off or reload the operating system, the computer will look first in your A drive, find the diskette with its boot disk
virus, load it, and make it temporarily impossible to use your hard disk. (Allow several days for recovery.) This is why you
should make sure you have a bootable floppy.

Macro viruses. These are among the most common viruses, and they tend to do the least damage. Macro viruses infect your
Microsoft Word application and typically insert unwanted words or phrases.

The best protection against a virus is to know the origin of each program or file you load into your computer. Since this is
difficult, you can buy anti-virus software that typically checks all of your files periodically and can remove any viruses that are
found. From time to time, you may get an e-mail message warning of a new virus. Chances are good that the warning is a virus
hoax.