While your nose picks up the chemicals that create aromas, it's your brain that actually tells you what you smell. That's one reason that patients who suffer from Alzheimer's disease and other neurological disorders sometimes experience a loss of their olfactory abilities [source: Doty].
What's even more surprising, though, is that mental illness can make you believe that you smell terrible, even if you don't. Olfactory Reference Syndrome is a psychiatric disorder characterized by a false belief that you have a bad body odor. Eighty-five percent of people with the disease report that they actually can smell their own imaginary stink [source: Phillips and Menard].