She watches him drop down into the lift, disappearing from her direct line of sight. She can't hear anything except the lift doors opening, and if she cranes her head far enough she can sort of see, but not enough, not really.

"I'm in here, come on!" That's the Doctor, from below, presumably to the plague-bearers.

"Don't tell them!" Cassandra hisses, having had enough running for one day, but she stays where she is and pulls the lever at the Doctor's command as he continues to encourage them to come closer. There's no way back up, no way out but through all those people. Nothing to do but trust in the man who'd already gotten her killed once. And maybe it was just Rose in there pushing a bit, but she very nearly did trust him.