• hidden
• self-copy
• harmful

Computer viruses infect system files and documents created by programs with macro capabilities. Some system viruses are programmed specifically to corrupt programs, delete files, or erase your disk.

Macro viruses spread quickly
Macros are simple programs that are used to do things such as automate repetitive tasks in a document or make calculations in a spreadsheet.
Macros are written in files created by such programs as Microsoft Word and Microsoft Excel.
Macro viruses are malicious macro programs that are designed to replicate themselves from file to file and can often destroy or change data. Macro viruses can be transferred across platforms and spread whenever you open an infected file.

Trojan horses hide their true purposes
Trojan horses are programs that appear to serve some useful purpose or provide entertainment, which encourages you to run them. But the program also serves a covert purpose, which may be to damage files or place a virus on your computer.
A Trojan horse is not a virus because it does not replicate and spread like a virus. Because Trojan horses are not viruses, files that contain them cannot be repaired. To ensure the safety of your computer, Norton AntiVirus detects Trojan horses so you can delete them from your computer.

Worms take up space
Worms are programs that replicate without infecting other programs. Some worms spread by copying themselves from disk to disk. They search for specific types of files on a hard disk or server volume, and try to damage or destroy those files. Other worms replicate only in memory, creating myriad copies of themselves, all running simultaneously, which slows down the computer. Like Trojan horses, worms are not viruses and therefore cannot be repaired. They must be deleted from your computer.
How viruses spread
A virus is inactive until you launch an infected program, start your computer from a disk that has infected system files, or open an infected document. For example, if a word processing program contains a virus, the virus activates when you run the program. Once a virus is in memory, it usually infects any program you run, including network programs (if you can make changes to network folders or disks).
Viruses behave in different ways. Some viruses stay active in memory until you turn off your computer. Other viruses stay active only as long as the infected program is running. Turning off your computer or exiting the program removes the virus from memory, but does not remove the virus from the infected file or disk. That is, if the virus resides in an operating system file, the virus activates the next time you start your computer from the infected disk. If the virus resides in a program, the virus activates the next time you run the program.
To prevent virus-infected programs from getting onto your computer, scan files with Norton AntiVirus before you copy or run them. This includes programs you download from news groups or Internet Web sites and any email attachments that you receive.

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