DODO: It’s nothing to worry about. His temperature’s a bit high, that’s all. It’s just a fever. It can only be a cold.

DOCTOR: Do you realise what this means, dear boy?

STEVEN: What’s all the fuss about? The man’s caught Dodo’s cold, that’s all.

DOCTOR: All? All? These people, this generation, have never experienced the common cold for the simple reason it was wiped out many generations ago before they were born. They have no resistance to it.