The “Smile Virus” is a fan-made horror story based on the “My Little Pony” (MLP) franchise. It’s part of a larger trend known as “MLP Infection” or “MLP Infection AU,” which includes various alternate universes where the characters are affected by different fictional diseases. The Smile Virus specifically is depicted as a condition that corrupts the characters into unrecognizable versions of themselves, often with a horror or zombie-like theme.