Similar to their biological brothers, computer viruses are designed to propagate, traveling from computer to computer to perform some mischief. That mischief may be as innocent as displaying a message that reads "This Computer is Stoned," or as fatal as wiping out every file on a hard drive. There are more than 48,000 known viruses, and 1,000 or so are active.

1. A virus consists of at least two parts: the replication code, which spreads the virus, and the payload, which is the prank or destructive part. Whoever wrote the virus inserts the virus code into an otherwise harmless program. The program—with the virus—is distributed through the Internet, on floppy disks, or even on commercial CD-ROMs.

2. Once you open the host program, the replication code is activated. The virus spreads copies of itself to other drives on your PC and to other PCs on the same network. Each of these child viruses becomes a parent virus and replicates even further.

3. A virus may remain dormant for months so it can spread without suspicion. It waits for a trigger, usually a specific date, to launch itself. If an infected computer boots or an infected program is launched under trigger conditions, the rest of the virus activates to deliver the payload. Typically the virus will destroy the boot record or files on your drive that have certain extensions.

4. Other viruses called worms replicate and spread with great speed. One such worm is an e-mail macro virus, such as Melissa or the Love Bug, which is distributed as an attachment to an innocent-looking message. Merely opening the message to read it activates the macro. A typical macro virus accesses your mail program's address book and sends copies of the infected message to everyone in the book. This begins a chain reaction, with each recipient perpetuating the virus. This in itself can be the macro virus's payload: jamming mail systems with so many messages that the systems are too packed to carry legitimate mail. Other macro viruses destroy files and boot records as well.

Illustration by Timothy Edward Downs