Meriam was a soldier on Rescension, who was injured badly during a Serfian raid. She was taken back to the human medic camp with the help of Donna Noble. As a result of her injuries, she was given an injection, which was supposedly a revitalising technique.
In Sergeant Lenova's office, she suddenly transformed into a hideous Serfian herself and was told by Lenova to kill Donna. But Donna in her turn explained that she was a human and not a creature and managed to turn the Serfian upon Lenova. The creature was killed, just like the rest of them. (AUDIO: Pest Control)