How can your nose signal a problem?

Your nose is part of your sinus system (ew!) and it can tell you if you're sick by a runny nose, congestion, and sniffles.

It can detect dangers, both inside and outside the body.

Explanation:

Outside of the body, of course, it can smell smoke and things like that, but if you're going into diabetic ketoacidosis, you'll smell a very sweet, fruity smell before you start to have seizures and other effects.