Killed By Death
Buffy is out patrolling, despite suffering from a flu bug. After a run-in with Angel, she collapses and is brought to hospital. After she is stabilised and admitted, she has a dream about a strange looking creature and a young boy - except the young boy is really in the kids ward with similar flu, and has also seen the creature. The boy, Ryan, says the creature is Death, informs her that children have died because of him, and then gives Buffy a drawing of the creature, which only kids can see. Initial research points to a more mundane source of the kids' death: Dr. Backer, who has used unorthodox cures in the past. However, Buffy discovers the slashed corpse of Dr. Backer, killed by an invisible creature that attacks Buffy.
Eventually Giles and Cordelia discover that it's Der Kinderstod - german for child death, a monster that can be seen only by children and also feeds on them. Buffy realises that she can see it only when she's feverish and convinces Willow to give her a small dose of the fever to make her sick enough to see it.