It is my understanding that lactose intolerance in infants is possible
but rare.

My memory from my readings (sorry can't site now, but multiple medical
texts)is that it was unheard of or almost unheard of short of the
following conditions.  In other words, rare enough to be a sort of
inborn error of metabolism.  There is too much lactose in human milk
for it to be common problem, if you believe in evolution.

Lactose intolerance in older children and/or adults becomes common,
depending on ethnic background, occuring more in areas where herd
animals were not used therefore exposure to milk ceased after weaning
from the breast.

Conditions where it can occur in infancy include:

prematurity (sort of a developemental thing, lactase being a late
forming enzyme during gestation),

dose related, or lactose challenges above normal - like too early
switching of sides and foremilk overload,

and

gastrointestinal upsets (infections) that have caused damage at the
cellular level where lactase is made in the bowel.



Carla (just north of Washington DC where we occasionally have those
alien lactose intolerant babies too)