Variant 7 was a type of drug created by the Daleks for curing the NFS plague that had engulfed the inhabitants of the galaxy.
It was part of an elaborate plan by the Dalek Empire to boost their numbers after the Great Catastrophe Variant 7 was given to the individuals suffering from the plague. It contained extra DNA embedded in it that remained dormant in the body. Once it was exposed to a certain form of radiation, this additional DNA converted the host into a new Dalek, allowing the Empire to create a massive army. (AUDIO: The Future)