Alicorns do not have impregnable immunity nor immortality, but permanent longevity. They are susceptible to pathogens just as well as normal Equestrians are. While their natural body systems are heavily reinforced to facilitate their boundless lifespan, they can become ill. It just so happens Twilight catches a nasty bug that has some... chaotic side effects.

Pre-read by: ALEXAROTH and Taranasaurus0.0.