Disco Fever is a disease caused by standing outside when it's raining. Known symptoms include fever, urge to dance, and talking in jives. So far there's no known cure for the disease.
In Episode 2, Read it and Sleep, Rarity was concerned that she'd catch the disease during the storm. After she and Applejack were kidnapped in Twilight Sparkle's house, Rarity tried to trick Twilight Sparkle by claiming that she had the disease and had to be carried to the hospital.