[Vahan]

The one where Slimer was befriended by a thing that appeared in the form of a Creepy Child.

The one where they become allergic to Ghosts.

Everyone in the city got turned into insectoid mutants.

[Razorgeist]

I believe the first one was "Transcendental Tourists" One of those short episodes from season 5. As for the 2nd well that was "There's something going around" not to sure about the 3rd one.

[mrmichaelt]

Third one you might be thinking of "20,000 Leagues Under the Street"

[Kingpin]

"Creepy" is going to be subjective, but the first one could be... ugh... Baby Spookums.