Aerobic gram negative rod. S. dysenteriae (group A), S. flexneri (group B), S. boydii (group C), and S. sonnei (group D).
Human to human and from food and water. It is also an STD in men who have sex with men (PubMed).
Ciprofloxacin, 750 mg once a day; or azithromycin, 500 mg once a day, both for 3 days (PubMed). Resistance is increasing to azithromycin in men who have sex with men (PubMed). And in 2015 ciprofloxicin resistance was imported into the US to cause a multi-state outbreak, another homeland security failure (PubMed). Azithromycin resistant strains are spreading across the world as an STD in MSM in the east coast of the US.
It takes less than 100 organisms to cause diarrhea, probably as low as 10.