A computer virus is a program designed to co-opt, hamper, or damage the functionality of a computer.
When the Wraith attacked Atlantis, Dr. Radek Zelenka designed a computer virus to upload to the Atlantis database in order to keep the Wraith from accessing the information. (SGA: "The Siege, Part 2")
When a group of Wraith tried to ally themselves with Atlantis, they downloaded a virus with the information they shared. They were able to steal the location of Earth and a means to improve their hyperdrive before the virus deleted the information. (SGA: "Allies")