Not sure if mentioned previously, but chickens are a major risk factor for contracting Campylobacteriosis, essentially gastroenteritis caused by the foodborne pathogen,
Campylobacter jejuni. Poultry and other avian species are natural hosts of this bacterium. It wouldn't surprise me to learn that the potential public health concern could play a role in the prohibition of backyard chickens.
While the manifestation of the disease is not quite as severe as that caused by toxigenic
E. coli, the prevalence of this illness is much higher.
Long story short, if any of you get chickens, I'd like to study you!