This might help address the question of how much breastmilk is needed to make
a difference.

It is a summary of a work by Paula D. Scariati, an epidemiologist at the
Centers for Disease Control and Prevention in Atlanta.  It was published in
Pediatrics, June 1997

Breastfeeding affords tremendous protection against disease, but breastfeed-
ing does not have to be exclusive to confer the benefits.  A new study, in-
volving over 1700 households with infants two-to-seven months old, found that
infants who were exclusively breastfed had half as many ear infections and
bouts of diarrhea (a leading cause of infant mortality) as babies who were
exclusively formula-fed.  Mothers with infants were classified as either
exclu-sively breastfed, in one of three categories of mixed breastmilk and
formula, or exclusively formula-fed.  Babies who received both breastmilk and
infant for-mula received benefits in direct proportion to the amount of
breastmilk con-sumed.  "We found a direct link -- the more the babies' diet
was breastmilk, the fewer infections they had," explains Paula D. Scariati, an
epidemiologist at the Centers for Disease Control and Prevention in Atlanta
and lead researcher on the study.  "The more breastmilk an infant receives in
the first six months, the less likely the baby will develop diarrhea or an ear
infection.  In a society where exclusive breastfeeding is not always
practical, it is good to know that supplementing with small amounts of formula
does not eliminate the protection afforded by breastmilk."