What are Computer Viruses?

Computer viruses are programs written by "mean" people. These virus programs are placed into a commonly used program so that program will run the attached virus program as it boots, therefore, it is said that the virus "infects" the executable file or program. Executable files include Macintosh "system files" [such as system extensions, INITs and control panels] and application programs [such as word processing programs and spreadsheet programs.] Viruses work the same ways in Windows or DOS machines by infecting zip or exe files.
A virus is inactive until you execute an infected program or 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 may save itself to your hard drive or copies itself to applications or system files on disks you use.
Some viruses are programmed specifically to damage the data on your computer by corrupting programs, deleting files, or even erasing your entire hard drive. Many viruses do nothing more than display a message or make sounds / verbal comments at a certain time or a programming event after replicating themselves to be picked up by other users one way or another. Other viruses make your computer's system behave erratically or crash frequently. Sadly many people who have problems or frequent crashes using their computers do not realize that they have a virus and live with the inconveniences.
What Viruses Don't Do!
Computer viruses can not infect write protected disks or infect written documents. Viruses do not infect compressed files, unless the file was infected prior to the compression. [Compressed files are programs or files with its common characters, etc. removed to take up less space on a disk.] Viruses do not infect computer hardware, such as monitors or computer chips; they only infect software.
In addition, Macintosh viruses do not infect DOS / Window computer software and vice versa. For example, the Melissa virus incident of late 1998 and the ILOVEYOU virus of 2000 worked only on Window based machines and could not operate on Macintosh computers.
One further note-> viruses do not necessarily let you know they are present in your machine, even after being destructive. If your computer is not operating properly, it is a good practice to check for viruses with a current "virus checking" program.
How do Viruses Spread?

Viruses begin to work and spread when you start up the program or application of which the virus is present. For example, a word processing program that contains a virus will place the virus in memory every time the word processing program is run.
Once in memory, one of a number of things can happen. The virus may be programmed to attach to other applications, disks or folders. It may infect a network if given the opportunity.
Viruses behave in different ways. Some viruses stay active only when the application it is part of is running. Turn the computer off and the virus is inactive. Other viruses will operate every time you turn on your computer after infecting a system file or network.
How to Prevent a Virus Invasion!

1. Load only software from original disks or CD's. Pirated or copied software is always a risk for a virus.
2. Execute only programs of which you are familiar as to their origin. Programs sent by email should always be suspicious.
3. Computer uploads and "system configuration" changes should always be performed by the person who is responsible for the computer. Password protection should be employed.
4. Check all shareware and free programs downloaded from on-line services with a virus checking program.
5. Purchase a virus program that runs as you boot or work your computer. Up-date it frequently.

Trojan Horses
A trojan horse is not a virus. It is a program that you run because you think it will serve a useful purpose such as a game or provides entertainment. Like a "trojan horse" it serves not as it claims, but to damage files or perhaps plants a virus into your computer. A trojan horse does not replicate or spread like a virus. Most virus checking programs detect trojan horses.