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.
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.
(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.