CRD is a disease that spreads very easy throughout a flock if left untreated. The first thing you need to do is isolate infected birds, as close contact spreads the disease. Then you have 2 options. You can cull infected birds, and burn the carcasses, or you can treat the symptoms. Keep in mind that a bird that has had CRD, will become a carrier after symptoms go away, and may get sick again when under stress.
Mycoplasma Gallisepticum bacteria.
Note: Not all symptoms need be present to have CRD, and if some of these symptoms are present, it is not a bad idea to isolate the bird, and treat for this before it gets worse, or spreads throughout your flock.
Chicks that are exposed to these carrier birds may develop a natural immunity to CRD.
Do NOT consume eggs from treated birds until at least 7 days from the end of treatments.