The Doctor was shaking, he was determined to face them, even without much of a plan. He knew he wasn't so well and nowhere near as able to do things as he normally would. But he still wasn't going to stop, even if it go him hurt or captured.

"No......, I cam here to find out what was going on, I still want to do that," He coughed badly and was shaking. He really knew that he was too ill to do this. But it didn't stop him from wanting to try,
He knew that Donna was worried about him, He was sure he had been in worse situations before. He knew he had even been stuck in where he was poisoned and still had to carry on.

"Donna, Just be careful," The Doctor was afraid he couldn't keep her safe as well.