I'm not sure if it's ever happened in the Pacific Northwest, but bird poop can contaminate areas with a pathogen that persists in the soil and causes a tuberculosis-like lung disease. I think there was even a town in the Midwest that had to be abandoned because of it. Bird poop is also a vector for various parasites that can harm you and your pets. (Plus it's gross.)

Edit: It's called "histoplasmosis".