Lactose intolerance is not a disorder.
It is actually more common in the world than
lactose tolerance.
Originally, people in the world were all lactose intolerant. Their bodies ceased the production of the enzyme lactase after an early age.
Their diets did not contain much milk anyways, so it was not a problem.
Somewhere in Europe, a mutation occured where lactase continued to be produced through adulthood.
Lactose tolerance is a dominant
allele, so the trait became very common in Western Europe.