Over eighteen thousand cases of Shigellosis a year are reported in the United States alone. This number may be twenty times greater due to many mild cases that go unreported. Most outbreaks of Shigellosis occur in areas where hygiene and sanitation conditions are very poor. Frequently an entire community may experience an epidemic that can reoccur every few years. Shigellosis is more common in the summer since warmer conditions are favorable for the bacteria to multiply to infectious levels. The people that are most likely to be infected are infants aged 2 to 4 years old and the elderly. Common outbreaks occur in childcare settings and in families with many small children. In developing countries Shigellosis is more common. In the United States the numbers of Shigellosis cases have steadily risen in the last 30 years from 6 cases /100,000 people in 1965 to 12 cases/100,000 people in 1995.