Just because they don't have symptoms doesn't necessarily mean they are not 'contagious'....or that they all are not carrying the disease anyway. Symptoms show when bird(s) immune system is weakened for any number of reasons.
Respiratory issues are tough, hard to treat without a cultured swab to know what organism is at play and thus what med to use. Not sure even meds work on anything but symptoms, and bird(s) can be a lifelong carrier. That is why I wouldn't bother treating, but I don't treat for anything other than dehydration and bugs .