In Blackadder Goes Forth, there's a scene where Blackadder is discussing security with Melchett and Darling. Part of the dialogue goes thus:
Darling: So you see, Blackadder, Field Marshal Haig is most anxious to eliminate all these German spies.
Melchett: Filthy Hun weasels fighting their dirty underhand war!
Darling: And, fortunately, one of our spies--
Melchett: Splendid fellows, brave heroes, risking life and limb for Blighty!
Darling: ...has discovered that the leak is coming from the Field Hospital.
Edmund: You think there's a German spy in the Field Hospital? I think you might be right, there.
Melchett: Your job, Blackadder, is to root this spy out. How long do you think you'll need?
The audience laughs knowingly at Blackadder's Field Hospital joke indicating that it's meant to be funny but I've never understood what's funny about it. Could anyone explain?