Thrush

"Thrush is a fungal infection of the yeast-like fungus candida albicans. Thrush can affect all members of a breastfeeding family. Mothers and babies can transmit thrush to one another, and fathers or partners can be infected through sexual activity with the mother. Candida albicans is the same fungus that commonly causes vaginal yeast infections  in women, and is often treated with the same medications."

- From "Common breastfeeding concerns: what is thrush?

Other sources:

[http://www.llli.org/faq/thrush.html "Is thrush causing my sore nipples?" - La Leche Legue International]

"Thrush in breastfeeding moms" - Babycenter.com

"Thrush resources" - KellyMom.com