A Virus was once a viral infection, a Worm was fishing bait and a Trojan Horse was Greek mythology. The personal computer has expanded those definitions.

Computer viruses are small bits of software code that are added to legitimate programs, modify the program and then replicate themselves. Once opened viruses can do extensive damage to programs and operating systems in a computer.

The earliest viruses appeared in the 1980s on floppy disks. Once the disk was inserted and the drive and running, the virus would find another program to attach to and do its damage. Because software programs were much smaller the viruses were spread from disk to disk. This factor minimized the spread of viruses – until the Internet became accessible to home computers. Then, users could go to Internet bulletin boards and download programs onto a floppy disk. If the program contained a virus, the computer became infected.

Email viruses are contained in an email or in email attachments. Often, the virus is activated when the user double-clicks on the email or the attachment. Once activated, the email replicates itself and automatically mails itself to up to 50 addresses in the user’s address book.

A Trojan Horse virus is a computer program that appears to perform one function and, in fact, performs a different and harmful function when launched. It generally does significant damage to the computer.

Worm viruses typically reside on a network. They find security holes in a computer and attach to a resident program. A worm can simultaneously do damage to a computer while slowing down Internet traffic and taking up bandwidth space.

As viruses evolved with the transition from floppy disks to CDs that protected programs from modification, hackers created viruses that loaded into a computer’s memory and ran in the background as part of a two phase action. Once the virus infected the computer – the first phase – the second damaging “attack” phase would occur, triggered by another computer event.