If you have already had breast cancer and had to have a
mastectomy (or had breast surgery for any other reason), you
might still be able to breastfeed.
How does breastfeeding help prevent cancer?
Part of the effect must be due to hormones. Your body has
highly altered hormone levels during pregnancy and lactation.
Presumably, hormones are also responsible for the fact that
breastfeeding mothers often experience a relief from symptoms
of asthma, diabetes, arthritis and some other chronic
diseases.
However, part of the effect seems to be more direct (I have
no idea how it works): research has shown that in cultures
where women only nurse their babies from one breast (because of
their traditional clothing style, which allows easy access to
one breast but not the other) they are much more likely to
develop cancer in the unused breast.