I learned that lesson in 2005. Brought in a Australorp Cockerel to be a breeder and because I trusted the 'friend' I was getting him from I just put him straight in. He was sick in 2 days with the most horrible smelling respiratory infection.
Smelled like putrid infection and rotting chicken flesh. That was when I learned the smell of Infectious Coryza. Once confirmed I had no choice but to process the entire flock of 8 hens making sure to dispose of the head, lungs, and trachea as directed by NC Agr. Sanitized the lot and burned the temp coop I had slapped together for them. I think I went a little overboard with the burning, but I needed a way to release some anger. Luckily they were far enough from the rest of my birds that I was able to keep my babies/pets. I did a self quaranteen on them for 6 months. None ever got sick and after talking to a state tester/vet at a show I knew they were safe.
Matt

Matt