I have only ever had lice, when I got my first chickens from a breeding farm. They brought them with them. I noticed it pretty quickly on their behavior. Used gun oil on the perches and the side holders. Since then I haven't had any. 4 years now. So my theory is, you'll bring them in with new birds.
Lice are parasitic insects found in the environment like any other insect; they're a nuisance with our birds, but easily treatable. And yes, it is always a good idea to treat new birds for lice as they will spread throughout the flock in time.