It's well-known that breast is best but mother's milk may have an ability
to change its composition to suit the needs of a sick infant.  The quality
of breast milk may be enhanced if a mother is exposed to a range of
viruses and bacteria while nursing.  The full article is at:

http://www.abc.net.au/science/news/stories/s877755.htm

This article also has links to the following three related items:

More warnings against 'excessive cleanliness', 2 Jul 2002

Peanut allergy may come from breast milk, 4 Apr 2001

Over 350 toxins found in breast milk, 12 Jul 1999