90% of the cases are Mycoplasma Gallepsum (chronic respiratory disease) or sinusitis.
No antibiotic in the world will cure them, time gets them better, but they will always be carriers of this and spread it to other birds. Problem is, your birds get better in 2-3 weeks, so everyone thinks, YAY they got better, well the did, but now they are exposing it to all the others.
Unfortunately, whacking them in the ol noggin is the only way to stop it. I know it sounds horrible and hard to do to your loved babies. But...think of the long term consequences.
Many states now require in addition to normal NPIP testing, that your bird are also MG tested clean too, to ship into their states.
This stuff is always been around, and is in every state, for some reason, most show symptoms in the fall though.
Just look at this board. Half the topics are help my bird has a snotty nose, swollen eye, sneezing, etc....well, short and narrow of it, that MG in most cases.
Hope everyones birds stay healthy!! Watch those weekend auctions too!! They are fun, but full of sick birds too.....