It Chapter 1 & 2: Every Monster Explained & Ranked
3. The Leper
Eddie's major fears are obviously linked to the idea of becoming infected by something and dying of an awful illness. He might have allergies but the anxiety is more rooted in the fact that his mother has turned him into a hypochondriac through over-zealous protection of him.
Somewhat inevitably, his fear is personified by It as a creepy, infected monster called the Leper, who is riddled with what turns out to be syphillis. He appears in the first movie to terrorise Eddie as he walks past the haunted house and drops his medication. The monster "helps him" pick up the pills and then chases Eddie into the garden where he meets Pennywise again.
The monster then reappears to the adult Eddie - proving his fears run deep - in the basement of the pharmacy where It makes him see his mother, tied up and then attacked by the Leper and its grotesque, phallic tongue. And not content with violating his mother, the thing does it to him too, until he gets an uncharacteristic surge of strength (which proves key in the end) and throttles it. At which point is vomits black bile all over him. Hell of a way to sign off, that.