In the wild they live in family groups, much like wolves, and have a similar social structure with an alpha, beta and omega.
The pet rat is the species rattus norvegicus, the brown or norway rat, whereas the rat associated (incorrectly, it must be said) with the plague is rattus rattus, the black rat. Domestic rats do not carry any diseases transmittable to humans and even wild rats are generally cleaner than most other wild animals, as well as the notable fact that it is impossible for a rat to transmit rabies to a person.
Rats have a very undeserved reputation, but this is slowly changing as more people realise what good pets they make.
There is a rat in the garden
Guy 2, "He is such a rat!"
Guy 3, "Wow, what a rat."
Girl 1, "Rat."
Guy 4, "You are all rats!"