I once had a mother who developed terribly sore nipples whilst
breastfeeding her baby and toddler. It appeared the toddler had hand,
foot and mouth virus and passed it to the mother's nipples. The baby did
not develop the virus. No treatment was required, other than allowing
nature to take it's course and keeping the nipples aired as much as
possible. It cleared within a day or two

Yvette O'Dowd
NMAA Breastfeeding Counsellor
Melbourne, Australia