A virus does NOT have to be destructive. Viruses can spread by diskette, network, email or Internet downloads. Occasionally, they are accidentally spread within packaged software products. Viruses cannot spread on their own and must be executed by someone to cause damage. Boot sector viruses spread when a user inadvertently boots his or her workstation from an infected floppy disk.
Macro viruses can spread by simply opening an infected document.
Mass mailing viruses utilize email to propagate rapidly, usually using social engineering tactics to persuade unsuspecting users to open the infected message. Once a virus is executed, it can impact memory, affect performance, modify data or delete files. Some viruses can wipeout hard disks or make them inaccessible. Viruses also result in significant wasted resources and costly downtime when dealing with the problem. The growth of the Internet and use of other online services has aided in the spread of viruses.