Hi and welcome to BYC - glad that you have joined us.
Yep, respiratory sickness can spread from chicken to chicken (unless its caused by fungus or an allergy) and chances are by the time we notice the symptoms, they have been suffering from whatever it may be for some time.
There are a number of schools of thought on what to do - I'm sure there may be others BTW
1. Take your chicken to the vet and go from there
2. Dose the whole flock with broad spectrum antibiotics or if you can get one that is for respiratory infections then all the better (they will not kill a virus, but can help stop secondary infections)
3. Give the flock vitamins / electrolytes in their water to help give their immune system a boost and see how they go.
Theres no reason not to begin on the vitamins ASAP.
Some respiratory infections are only as harmful as humans getting a cold, others a lot more serious. Assuming your flock survive and get over the illness, bear in mind that they may be carriers of the illness for life and will pass it to new flock members. It would be unwise to sell or give any of your flock to other chicken keepers in the future.
I'd suggest further reading on the subject as it can be complicated.
All the best
CT
