There are many different types of Shigella. One type, Shigella sonnei, causes over two-thirds of the shigellosis in the United States. A second type, Shigella flexneri, accounts for almost all of the rest. Other types of Shigella are rare in this country, but are important causes of disease in the developing world. Most Shigella infections are the result of the bacterium passing from stools or soiled fingers of one person to the mouth of another person. This happens when basic hygiene and hand washing habits are inadequate and can happen during certain types of sexual activity. It is particularly likely to occur among toddlers who are not fully toilet-trained. Family members and playmates of such children are at high risk of becoming infected.