Lactose intolerance

Lactose intolerance is a condition found in many adults where they lack an enzyme that breaks down lactose, a sugar found in the milk of all mammals. Unlike other complex sugars, which can be broken down in the body, lactose must be broken into its smaller components in the intestines before it can be taken into the bloodstream. Otherwise, it is as undigestable as cellulose. Unlike other enzyme deficiencies (such as the one that prevents the body from processing glucose, which is incredibly rare), lactose intolerance is very common even in persons of European ancestry, and is almost universal in persons of heritage where milk is not a regular part of the diet, such as Africa and East Asia.
However, lactose intolerance is almost unknown in children, who normally take in lactose as part of their mother's breast milk. It appears from an evolutionary standpoint that early humans lost the ability to digest milk sugars after infancy. However, as certain cultures developed cow milk as a viable food source, those persons who could continue to digest it into adulthood were favored.
If the lactose is not broken down by enzymes, it is perfectly digestable by gut bacteria, which quickly break it down into methane gas. As a result, the gas quickly builds up in the intestine, usually leading to flatulence and diarrhea.
Lactose tolerance is easily treated as those who have it can simply avoid dairy products such as milk and cheese. However, a synthetic enzyme, sold under the trade name Lactase(TM), can be taken in advance of eating dairy products and replaces the enzyme that is otherwise produced by the body, avoiding the symptoms.
Lactose intolerance at NIH
Lactose intolerance at Wikipedia
Lactose intolerance at Mayo Clinic