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