TSR Terminate and Stay Resident. TSR viruses usually design a method by which they are put into memory when the computer is booted, and then run until the computer is shut down. A modified version of an original virus. It's not uncommon to see a virus changed, and often damaged, by other virus authors over time. Virus A file that is designed to copy itself, and sometimes do other operations, like display strange messages or delete everything on your hard drive.
Some viruses are only within a company while others are spread out all around the world.
Once installed, it corrupts certain program files to pass its package on as part of the program's normal operation, e.g. by trying to e-mail itself to all entries in one's addressbook. A compressed file with the extension of .zip. Zipped files may contain viruses. Make sure your antivirus program scans for viruses in compressed files.