Steroids (especially short term) are OK with breastfeeding (see Briggs,
Friedman and Yaffee)

EBV infection (which is the cause of mono) is very common in young children
in more crowded living conditions. The kids usually do just fine - it is
generally passed of as just another virus. When you get it older is when you
generally feel quite sick. The baby is already exposed to the mother so I
see no benefit in stopping nursing (It is not listed in the Redbook (AAP) as
a cause not to breastfeed.