I caught the film Mulberry St a while back. It's an enjoyable but a little cheesie (forgive the pun) horror film that follows the model of the typical zombie outbreak flick but this time it's some sort of virus that changes people into rat-men.
This idea is just too good to pass up on in a fantasy campaign that has were-rats. It let's a DM use a classical D&D monster without having to rewrite the monster descriptions for the campaign. Were-rats are far more capable opponents than are zombies capable of using cunning and even moving about unnoticed now and again. If a DM plays up the eventual were-rat swarm attack the players may even think they are dealing with a zombie apocalypse, imagine the look on their face when the clawing gnawing horde turns out to be were-rats instead of zombies.
Lycanthropy kills demi-humans instead of infecting them with a curse. So a demi-human vicitm is a great way to introduce the situation to the PCs.
The party finds a wounded elf " I was walking past the graveyard when they came at me from the shadows. I thought they were people I was wrong...they bit me..."
Have the corpse stolen. if any player implies the corpse walked off run with that or even go so far as to have the were rats covering up their slayings as much as possible.
Does Lycanthropy work like rabies on inflicted demi-humans? If so the "rage" zombie scenario is easy to borrow to mask what is coming.
A number of people can disappear, be bitten and sickened and it keeps leading the party back to freshly dug up graves, a few recent victims can even be found being consumed by a swarm of rats and there are signs something walked away leaving bloody footprints and then wham...Were rat apocalypse.