S05E01: Something's Going Around

Peter falls ill, and Louis asks a doctor (Stanley Ralph Ross) he met two days ago to come and take a look. It turns out the doctor has sent them potato chips, which makes people allergic to ghosts, and only Peter has eaten enough of them. The doctor gives them more chips. What the Ghostbusters don't know is that the doctor is a ghost and they have to develop resistance to the allergy.