What nutrition is given to infants who are lactose intolerant? I understand, in today's day and that age we have non-lactose formula milk, but what happened, before lactose-free milk was created, to those who suffered from lactose intolerance (Native Americans, Asians, etc.)?
The lactose intolerance Wikipedia page explains the problem fairly well, so I'll refer you to that for a more detailed explanation.
Briefly, the most common cause of lactose intolerance is primary lactase deficiency, which affects the majority of the world's population. This only affects adults: the majority of people do not produce lactase as adults.
Congenital lactase deficiency, instead is a very rare, autosomal recessive genetic disorder that prevents lactase expression from birth.
Congenital lactase deficiency (CLD), where the production of lactase is inhibited from birth, can be dangerous in any society because of infants' nutritional reliance on human breast milk during their first months. Before the 20th century, babies born with CLD often did not survive, but death rates decreased with soybean-derived infant formulas and manufactured lactose-free dairy products. Beyond infancy, individuals affected by CLD usually have the same nutritional concerns as any lactose-intolerant adult.
A couple of good references:
Lactose intolerance in infants, children, and adolescents. - Heyman, Pediatrics 2006
Genetics of lactase persistence and lactose intolerance. - Swallow, Annu Rev Genet. 2003
In fact, being lactose tolerant is the mutation. Adult mammals are normally not lactose tolerant, only the still milk drinking babies have the lactose gene activated. Humans developed this mutation earliest in the Neolithic.
Lactase persistence (LP), the dominant Mendelian trait conferring the ability to digest the milk sugar lactose in adults, has risen to high frequency in central and northern Europeans in the last 20,000 years. This trait is likely to have conferred a selective advantage in individuals who consume appreciable amounts of unfermented milk.
(citation from Burger et al., PNAS, 2006)
So in fact, people who are lactose intolerant are just plain normal. There is normally no reason to feed babies lactose-free milk. The rare cases where you need to do this are very well described in the post of nico.