The Doctor tries to warn the nurses, but can't lose sight of Cassandra/Rose, so he takes off after her.

A moment later, there's the sound of an electrical explosion, and every cell opens up, disgorging thousands -- hundreds of thousands -- of disease-carriers into the narrow passageways.

Cassandra swears she didn't do it, and right now, the Doctor doesn't have time to argue the point. His first priority is to keep Rose safe, and that means, as it usually does, that they have to run.