(Lilith has just gotten off the phone with a colleague who supplies her with lab rats)
Lilith: Peter's come down with something. He thinks he caught it from the rats.
Frasier: (anxious to have Lilith be on her way) Oh, now, nobody ever got anything from a rat that wasn't resolved in a day or two -- off you go!
Frasier, of course, refers facetiously to the Black Death of the 1340's, one of the deadliest pandemics in human history, widely thought to have been Bubonic Plague. It's rapid spread was attributed to fleas on rats. The total number of deaths worldwide is estimated at 75 million people.