According to my understanding of immunology, breastmilk should contain
antibodies to all infections that mom has antibodies to, and new ones as they
are passed to her by the baby.  Human milk contains mainly secretory IgA,
which protects mucous membranes (ports of entry for germs).  The fetus gets
IgG through the placenta before birth.  IgG is the first class of antibodies
produced in the body after infection, and does the nitty-gritty initial germ
fighting.  These are the antibodies that only persist for around 6 months in
the infant's body.  The reason that people do not lose their immunity to
naturally aquired diseases is that the b lymphocytes that produce antibodies
develop into "memory cells" for each specific illness.  1 disease per clone
(cell and all it's decedents).  These cells keep reproducing at low levels,
until the germ shows up again, then they repro like crazy and start pumping
out huge quantities of antibody.  Other classes of antibodies have slightly
different functions, and work slightly differently.  So, the answer is that a
nursing toddler is still protected from the entry of the germ, still has
support for his immune function through the non-specific immunities of
breastmilk, and will have protection from responsive immunization as well (mom
making antibodies to germs that enter her nipple from baby;s saliva).  But the
IgG from the womb will have worn off, and that protection will be decreased.
So, it is less likely that the child will get the chicken pox, and if the
child does contract them, the case will likely be milder.
Catherine Watson  Genna, IBCLC   who loves the way things work.