The healing zones were vast, city-sized medical complexes developed by the Daleks for the treatment of the NFS plague. Built quickly with geo-forming, the healing zones took in thousands of patients and were the only places that administered Variant 7. (AUDIO: The Survivors. The Demons)
The hospital set-up was merely a ruse. The healing zones gave off massive amounts of radiation that reacted with the implanted DNA that was administered through Variant 7, mutating the recipients. Within the heart of each healing zone was a Dalek reproduction factory. Those factors made the healing zones breeding grounds for a massive Dalek army. (AUDIO: The Warriors, The Future)