When the Ghostbusters originally encountered the Ghoul it was a Class 3 terrorizing a chemical plant. When the Ghoul was originally trapped, some chemicals had gotten onto the Ghostbusters and absorbed the Ghoul's ectoplasmic energy, which caused the Ghost Trap and the P.K.E. Meter to label the Ghoul as a Class 2 instead of a Class 3. The experimental chemicals were designed to absorb a wide spectrum of energy. The Ghoul's ectoplasm and the Proton Streams sparked the chemicals and set off the reaction.
The chemicals caused the Ghostbusters to suffer bizarre disfigurements and body parts grew on their bodies (for example, Ray Stantz had an eye growing on his chest). However, it would eventually need more ecto energy to stay alive. After days of medical attention, the slime coats separate from the Ghostbusters and reconstruct the Ghoul. The Reconstructed Ghoul attempted to eat Slimer in order to survive. Unable to trap it, since it was not a ghost, the Ghostbusters overloaded the Reconstructed Ghoul with Proton Streams. The gamble paid off and it imploded, taking out most of the wing of the hospital. A week later, the team was discharged. Somehow, their bill was the same balance as their medical bill and cancelled each other out.