I think I figured out who was feeding the rats and where the flu started on the WD.

I think the young guy with the glasses who was the first one to die of the flu was feeding the Zombies. In that same episode while jabbing the Zombies on the fence they notice some had blood comming out of their eye and one of them said I havn't seen that before.

Then last week when they were all in the VET school and were being chase by Zombies they all had blood comming out of their eyes and one of them said they have the flu. So they busted out the door and took on those zombies rather then face the ones with the flu. The guy who died first was the one feeding the zombies and he got sick and pass it on to the others.

Sorry don't remember his name.

My money is on the governor.

governor was feeding them the rats. he's amassing an army of walkers.

although the flu explanation seems spot on.