The Virus Orb is a small device capable of keeping the A't'trr in stasis until they could be moved to a suitable new homeworld.
The orb was created over 100,000 years before its discovery by SG-1, by the inhabitants of P5C-353 after they discovered their planet was going to be destroyed. They constructed the orb and put the embodiment of their civilization, composed of countless intelligent entities at the microscopic level, inside of it. (SG1: "Message in a Bottle") These entities are called the A't'trr. On the outside of the orb was a written request to any traveler that might find it, to take it to a new world.
The A't'trr would wait in stasis within the orb until the orb detected it had been brought into an atmosphere of a living world. The orb then would attach itself to the planet by harpoon-like devices and remain there as the contained entities spread throughout the world.
The Orb later fell into the hands of SG-1 and the SGC. Their attempts to send it back ended it in disaster as it consumed the walls of the Embarkation room of Stargate Command, infecting random personnel and even used Colonel Jack O'Neill as a means of communicating with the SGC where it informed them of its intentions to grow and consume Earth itself.