GROSS! Warm winter = Rats.

Ok, if you ever get a chance to read Rats, Lice, and History by Hans Zisser, you should, because it's a great read.

Rats are so very very nasty. And warm winters = more rats.

Buffalo, NY is dealing with what is being called an infestation- EWWWW.
"Normally, a long hard freeze in winter kills off at least some of the rat population. But the balmy days of December and January mean communities battling rat infestations in metropolitan Erie County won't be able to rely on the weather for a helping hand. " LINK

For just a few reasons rats are totally utterly horrible and dangerous to humans, check out this list of just a few of the deadly diseases associated with rats. (From the CDC)
Hantavirus Pulmonary Syndrome
Murine Typhus
Rat-bite fever
Salmonella enterica serovar Typhimurium
Leptospirosis.
Eosinophilic Meningitis

And let's not forget my personal favorite rat-spread disease, the Bubonic Plague.