Lactose intolerance is an intolerance to milk due to an inability to digest the sugar lactose. This is caused by inadequate production of the digestive enzyme lactase, and commonly results in severe abdominal pain, gas, and diarrhea.
Lactose is found in most types of milk, especially cow and human milk. Most infants and small children have higher levels of lactase than adults to aid in digesting their primary food, breast milk, and therefore lactose intolerance (LI) is most likely after a child reaches four to five years of age, although it may appear much later in life. Most adults have some degree of LI: Asian, native Australian, and many African populations tend to become intolerant as they get older, while European, Arabian, and East African populations are less likely to be LI*. (This is an excellent example of microevolution in humans; those populations that relied heavily on milk for centuries have slowly developed to retain the lactase into adulthood.)
Symptoms of LI may include abdominal bloating, excessive intestinal gas, nausea, weight loss, diarrhea, and abdominal cramping. LI may result in intestinal diseases such as celiac sprue and gastroenteritis. There is no cure, except to stop drinking cow milk and eating milk byproducts such as cheese and ice cream.
LI can be managed without eliminating milk entirely. There are dairy products that have been treated to 'pre-digest' the lactase; lactose free milk is commonly available in most grocery stores. Many LI individuals can handle lactase at low levels, particularly when eaten with other foods. Some dairy products, such as yogurt and kefir are naturally low in lactose, as the bacteria that work to curdle them feed on the lactase. Other types of milk, such as goat milk, are also naturally low in lactase.
*
Africans (as a whole): 10-30% retain lactase into adulthood.
Orientals: 0-30% retain lactase into adulthood.
Northern Europeans: 80% retain lactase into adulthood.
Haviland, 1994, Anthropology
This is almost certainly due to centuries of Europeans drinking fresh milk. Other cultures that traditionally sour their milk (or did not raise cattle) did not need to evolve to extend their lactase production into adulthood, as the souring process reduces lactose.