Mysterious Girlfriend X.

Akira is a normal high school boy, but he nevertheless impulsively licks some drool left by the mysterious transfer student Mikoto. The next day he falls asleep with an inexplicable fever. After five days Mikoto suddenly comes to visit, and makes him consume her drool. The fever instantly vanishes, and Mikoto explains that he's suffering from "love sickness." She can transmit emotions through her drool, and the two of them are now bound together.