A Bacteriophage was a bacteria that infects and replicates within a bacterium.
The Wraith were aware of a bacteriophage they called The Little Replicator because it reproduced nearly as fast as a Replicator. According to the Wraith scientist Ember, this bacteriophage was very aggressive and very effective at colonizing bacteria. In its natural state, the Little Replicator was fairly rare and reasonably fragile, but once it finds a host, it is much harder to kill and enhances the effects of the bacterium in question. The Little Replicator is extremely persistent to the point that the Wraith have had to destroy entire hive ships to stop it. Eventually, the Little Replicator will mutate to the point that it will become fatal to living organisms. In Wraith, it causes a fatal infection in their lungs and spreads so fast that not even the Wraith ability to heal can keep up. The speed of its mutation depends on the strain of the bacteriophage and the strength of it.
In 2010, the Atlantis expedition inadvertently picked up a Wraith solvent infected with the Little Replicator. The organism, called alflageolis, began spreading throughout Atlantis, attacking and destroying all of the plastics within the quarantined area and resisting all efforts to stop it. The Wraith Ember eventually recognized the Little Replicator as being responsible, identifying it as a relatively weak strain, giving them potentially a week to fix the problem before alflageolis mutated to become deadly. However, Ember and the team working with him, including Doctors Radek Zelenka and Carson Beckett had an extremely difficult time coming up with a way to kill alflageolis.
Eventually, Ember came up with the plan to create a more voracious bacteriophage to attack the Little Replicator and its host bacterium. Ember was able to insert a kill switch by adjusting the new bacteriophage's genes related to reproduction to cause it to go inactive when the cell tried to divide. Ember was able to find a way to introduce this new bacteriophage to the equation by creating a protein to carry it that alflageolis would absorb. After Ember's plan was approved, the new bacteriophage was spread in aerosol form by the combined efforts of the science and military personnel of Atlantis. Ember's bacteriophage proved successful in killing the Little Replicator and the alflageolis before it could mutate to be fatal. (SGA: "The Third Path")