Shigella is a genus of bacteria that can cause sudden and severe diarrhea (gastroenteritis) in humans. Shigella thrives in the human intestine and is commonly spread both through food and by person-to-person contact. A Japanese scientist Kiyoshi Shiga discovered these bacteria over 100 years ago. Shigellosis is the name of the disease that Shigella causes. The illness is also known as "bacillary dysentery."