When an infected file is executed or a computer is booted from an infected disk, the concealed virus is executed. Usually, it lurks in memory, and is waiting to infect the next program that is run, or the next disk that is accessed. In addition, many viruses also perform a trigger event, such as displaying a message on a certain date, or deleting files after the infected program is run a certain number of times. The majority of viruses are harmless but some viruses, however, are menacingly. They lead to system crash, file corruption and data lost. A common virus in campus is WordMacro.Cap which infects Microsoft Word document and propagates through e-mail attachment. Please note that your computer hard disk and diskettes can be infected with Computer Virus by sharing infected floppy diskettes and documents. Virus cannot infect files in a write-protected diskette.