Rats are aggressive and will frequently attack on sight, attacking one rat seems to increase the chance of others joining the attack. Witches, however, will not be attacked by rats and may move among them freely.
If you are hit by a rat you will become Diseased, if you are already Diseased it will worsen your illness. There are 4 Diseases Maroon Fever, Big Pox, Frontier Flux, Land Legs, you can only have one at a time.
Rats will "stack" disease. The more you are bit, the more severe the disease will be.