Yes, your other chickens have been exposed already, and likely they will also fall ill in a few days to a few weeks- depends on the incubation period. Most chicken respiratory diseases are highly contagious. Some are severe, some are mild. Some are viral, some are bacterial. Unfortunately- it can be hard to tell what disease it is. ILT, MG, IB, coryza- ect They all cause sneezing/coughing to some extent. Some have high mortality (death). Next time- inspect your possible purchases thoroughly (including just standing there and listening to them for 5 minutes) before bringing them home- don't bring home sick chickens. Next time quarantine your hopefully healthy purchases- MINIMUM 2 weeks. Do not mix with your existing flock.
Now- some options are besides immediately removing all new birds into quarantine--- 1) cull someone sick and send them to your state lab- ID the bug so you will know what the long term consequences are to your existing flock, as well as help with treatment plan if there is one 2) get a local vet to start the process and suggest a disease based on clinical signs 3) treat yourself- guessing the diagnosis bases on clinical signs- look up signs of MG, ILT (often bloody material coughed up, death is common), IB, Coryza (usually smells bad). The viral diseases will not be helped by tylan, sulfa drugs, tetracyclines or other antibiotics. The bacterial infections may be, depending on what you use. Carrier states are common- which is why ideally- even if you pick option 3) and start an antibiotic, you still do option 1) to get your diagnosis in a week.
Another thing to do is to call the seller- let them know, find out what they know (or don't) about any recent illnesses. One hopes that you bought from an ethical seller, who will be honest with you.
