Buffy's just plain worn out y'all. Weakened emotionally and physically by her struggle with Angelus as well as a nasty flu she's laid up in Sunnydale General. As the fever rages she'll have to confront her childhood fear of hospitals (for very good reason it turns out) as well as an invisible, murderous creep (eye)stalking the children's wing.

The Buffy Boys are leaving balloons, chocolates, and non-committal 'Get Well Soon!' cards in S02E18 "Killed by Death".