Cassandra was back, in a wonderfully poigtant and touching fashion, taking over Roses' body for most of the episode, Rose herself took mostly a backseat to affairs as Cassandra took center stage causing as much havoc as she could, eventually taking over The Doctor's body for a breif period, it did lead to some funny exchanges. Then she discovered how lonesome one of the Flesh Zombies were when she entered one of them, and helped The Doctor create a potential vaccine.
The epsiode's themes on cleansness and new life was abundant here, the Flesh having never known a human touch, and when they did touch anyone, they infect and kill them. The conclusion with them all being cured of their virual strains, and getting the chance to live for the first time (they had only been bred with every deisease on Earth for drug testing purposes for cures etc)
The Cat Maidens getting theirs at the hands of The Flesh during their rampage was extrmely graphic, with horrific scarring. Cassandra finally entered the body of her half life man servant who was already dying from a heart attack, so The Doctor took her back into her own past to say to her old human form how beutiful she was, realting to an event Cassandra told The Doctor earlier in the episode.
Cassandra: This form of skin at the back was once my front, the lower region from my back is now my front
Rose: Oh right, so you're talking out of your ar-
Cassandra: Ask no more questions