Rats: History.

Terrance Powell: "The campaign to get rid of rodents goes back to the dark ages when plague was spread by fleas on rats."

I really like this guy. He's apparently intelligent, he doesn't seem to be paralyzed by irrational fears, he knows his stuff! The rats, actually, a lot of wilderbeasts, carried the FLEAS that spread the plague. Thanks to man's innate desire to live in complete and utter filth (hasn't changed much, really), the rats were able to live, unnoticed, in the dwellings of man. The flea hopped off the rat onto a heartier, healthier meal, and voila, "plague." Recent research on the topic even incriminates us for the spread of the plague: It spread too fast to be transmitted by the fleas and it spread too far and to too cold of regions (where the rats wouldn't have travelled).
It also doesn't hurt to note that he said "the campaign to get rid of" - Yep, we're still trying but until we stop catering to their every whim and dream, it seems they'll just continue to be a "problem." Not that I mind. I kinda like the little buggers :o)

Commentator: "The movie Willard plays upon a fear of rats that has paralyzed humanity since biblical times."

Oh, good. For a moment there I optimistically thought stupidity was a recent occurrence."