The type and extent of the damage varies from virus to virus, from simply displaying a graphic to corrupting the data on your hard drive. Even a simple virus that does nothing but replicate itself can cause system performance problems by taking up valuable storage space and memory. These are boot sector, program files, and macro viruses. When you turn on your system or reboot, the virus copies itself onto the boot sector of your hard drive. When an infected program is started, the virus loads into your memory and can go on to infect other programs.
Macro viruses are new to the scene but are becoming the most common type of virus found today.
These viruses attach themselves to macro commands that are themselves attached to documents and worksheets. Boot sector viruses infect the computer when the computer is booted from an infected diskette. Program viruses are passed with infected programs. Macro viruses are passed with infected documents, either via e-mail attachments, floppy disks, web downloads, file transfers or cooperative applications. Fortunately, for now, macro viruses are application-specific because they must be written in the language of, and use commands that are understood by, that particular application.