Artificial intelligence entity
A powerful alien force from a distinctly alien planet, capable of piggybacking through the Stargate during an outgoing wormhole and embedding itself within Earth computer technology -- as well as the human brain. It is distinctly intelligent (as well as vengeful) and wishes self-preservation for itself and its entire race.
According to the entity, radio waves transmitted back through the Stargate destroy the species.
Entity - An artificial intelligence travels through an outgoing wormhole and infests the S.G.C. computers, eventually traveling into Major Carter.