What about pneumonia?

Dear Dr. Sadie,

I have a house that's right next to a lake. Every morning, I wake up and drink my coffee out on the deck and look out at the lake. It's lovely, but I've been struck by a thought of late. It's quite cold each morning, and certainly damp, yet the ducks are out there, happily hunting for breakfast. I was wondering, why don't they get colds.

- Curious Bird-Watcher

Maybe because ducks have thick coats of duck fur. I mean like thick feathers. That helps them not get sick.

When the feathers fall off, the duck gets sick, but when the feathers stay on, the duck isn't sick. The feathers keep them nice and warm when it's cold out.