What is a virus?
Simply defined, a virus is a self-replicating piece of software. Many viruses, however, are additionally configured to harm your computer by altering the way it works. Depending on the virus, these alterations may be as benign as displaying a message on a certain date, or as destructive as erasing your hard drive.
When an infected file is executed, or the computer is started from an infected disk, the virus itself is executed. Often, it lurks in memory, 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, displaying messages or pictures, or doing nothing at all. Other viruses are annoying, slowing down system performance, or causing minor changes to the screen display of your computer. Some viruses, however, are truly menacing, causing system crashes, damaged files and lost data.