Acne (2000)

Franny and Zoe wake up one morning ready to go on a high school field trip. Their plans quickly change when Zoe drinks contaminated water and his head becomes a huge pustule. When Franny comes to aid him, Zoe turns her into a monsterous "zithead." Soon the two of them set off on a bizarre journey, mutating other teenagers, encountering strange suburbanites, and developing a very unusual craving for chocolate. The army starts a statewide manhunt for the infected adolecents as they struggle to remain concious, try to figure out what's happened to them, and draw nearer to a terrifying fate.