If you mean the shopkeep of the potions place in Whiterun telling you you're sick, she just says that by default. If you mean general NPCs, check your active effects for any negatives to see if you're diseased and use a cure disease potion or a temple altar to cure yourself.
On my woodelf I used to get told that ALL THE TIME. One of the old lady's in Whiterun kept telling me that every time i passed her... no diseases on me and with the rested state on me. I finally just killed her to make her shut up.