A virus is a program that runs and spreads by modifying other programs or files. A virus cannot start by itself; it needs to be activated. Once activated, a virus may do nothing more than replicate itself and spread. Though simple, even this type of virus is dangerous as it can quickly use all available memory and bring a system to a halt. A more serious virus may be programmed to delete or corrupt specific files before spreading. Viruses can be transmitted via email attachments, downloaded files, instant messages or via diskette, CD or USB devices.