Well a lot of people give their chickens medications for chronic respiratory disease such as Tylan, Denaguard(?) and some give Duramycin or whatever chicken antibiotic the feed store sells.
I don't know what to tell you but you might do a byc search for those medications to see if that is what you have in mind. Jefferslivestock.com sells a lot of meds if you can't find them in your feed store.
Just to let you know that the sniffling sneezing draining nose thing with chickens is a disease usually. This can result in a carrier status even after the antibiotics (and some of the diseases can even pass from mama to egg). If you intend to be a breeder you might send a chicken that is deceased for necropsy or ask a vet if they can swab something to find out what it is???
One lady on here thought it was chronic respiratory disease but sent a chicken for necropsy and it was internal parasites! (Totally curable.) She was so glad she didn't cull before testing.
But if you are just interested in chickens for your family some just treat and retreat as symptoms arise.
Breeders will tell you to cull cull cull for respiratory symptoms in chickens and then eventually you might eradicate the sickness. I would consider separating the sick ones from the rest of your flock. If you want to possibly eradicate and cull, separate. If you want to just let it go and give meds to everyone, you might just let the survivors be together. However, if you bring in new chickens they might get it pretty quickly even though no one in the flock appears sick. That's just how these things can be.
And I have read some writing posts on here that treat their backyard flock and it never came back...things are fine but they didn't introduce new ones and they close the flock. No in, no out.
I know, more info. than you wanted. But here is a link to the common respiratory illnesses and diagnosis charts at the bottom:
http://edis.ifas.ufl.edu/ps044
