Viruses are small software programs that "attach" themselves to other programs. Once a program is run, the virus begins to replicate itself and attach itself to other files resident on the machine.
Viruses do many different things. Many lay dormant for a long time and then can cause problems on your system when a certain file is used or on a certain day. They can be like little mischievous gremlins or they can cause a lot of damage, like wiping your hard disk. Others make themselves present every time you boot your system. Viruses are also transmitted and shared by attaching themselves to outbound e-mails or files saved to storage devices such as Portable USB drives. This allows them to spread much like a communicable virus from person to person.
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