Breed or genetics sickness or disease?
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It's hard to say whether it is an environmental problem (from dust in feed or bedding, mold, or poor air circulation) or an infectious problem such as a virus or bacterial disease without testing or having a necropsy done. Fortunately, it seems to only affect your meat birds, which I assume you are going to butcher soon. Infectious bronchitis can be very common with sneezing the usual symptom, but with mycoplasma (MG) there can be mild strains or severe ones, and it can pass through some of the hatching eggs.