The only way to maintain a healthy aviary is (1) to start with a healthy flock of birds and then (2) to prevent the existing flock to be exposed to disease-inducing pathogens.
Before adding birds to the existing flock, the breeder needs to keep the newly acquired bird(s) in total separation from the other birds, in quarantaine facilities that do NOT share the same air as the other birds in order to prevent airborne contaminants from accessing your existing flock. Recommended period for quarantaining birds is three to six months, especially they come from a questionable or unknown source. Furthermore, it is strongly recommended that birds receive a thorough "Bird Health Check" before allowing them contact to your other birds. (Please also check out "Quarantaining Your Bird").