A virus or worm attack on a computer can occur in a variety of ways. Viruses have both "trigger" events and "attack methods" that define how they do damage to computers and files.

Trigger Events

What causes a virus to "go off?" The most common way a virus is triggered is by the user : When someone receives an attachment in an email, they open it unsuspectingly, and the virus is triggered. The damage is done almost immediately and sometimes even shutting down the computer cannot stop the attack from continuing when the computer is restarted.

Some viruses infect computer files and lay dormant until a certain set of conditions occur, such as a special date (the Michelangelo virus waited until the 500th anniversary of Michelangelo's birthday) or a certain file is used again since the infection occurred. 

Additionally, the newest viruses can be triggered without any user activity or special conditions - but simply by reading an email or a visiting a web site. These viruses may be embedded in graphics, multimedia files or other downloaded components that happen simply by "receiving" or "loading" the files onto your email or browser software.

Attack methods

What do viruses do when they attack a computer? Viruses can perform many different attacks on a computer and its files. While most viruses cannot harm computer hardware such as a monitor or keyboard, they can harm memory chips and flash memory, especially those viruses that attack Palm Pilots or similar products that store their files in memory and not on a hard disk.

The most common viruses attack stored files on your hard disk. Macro viruses not only attack files and make them unusable, but they then mutate themselves before spreading, making detection even harder for the antivirus software or technician. The type of damage these viruses perform is usually file deletion or corruption , making documents, spreadsheets or graphic files useless.

File infector viruses attack programs rather than data. They can alter the nature of executable (.exe) files or your system's operating software (such as damaging registry files) so that the user is unable to launch programs such as their word processor or web browser. These program are particularly pernicious because the virus prevent the user from running programs or getting onto the web to disinfect their system!

Boot sector viruses attack the start-up area of your computer. This type of virus simply makes it impossible to load your operating system and start your computer. Variations of this virus allow the user to start their computer, but because the virus resides in memory, it can replicate itself to other disks on the network or diskettes and zip disks the user may use and give to other people.

Retro Viruses are actually viruses that attack antivirus software! These viruses will disable your antivirus software or cause the protection files to become corrupt so that the user becomes vulnerable to the most common viruses or attacks.

Can a virus be stored on a backup media?

Yes! A virus that has infected a file or program will be backed up with that file onto tape or zip disk. When a user gets a virus, they will proceed to disinfect the system and repair the damage. However, if they restore files from their backup media, they may unknowingly be restoring the virus with it - and the cycle will start again. It is important to make sure your system is virus free before making a backup to avoid this conundrum!