I looked in the "immune system booster" Transfer Factor, and it sounds
like it is NOT something I'd be inclined to spend any of our hard earned
money on.
 
To see what bits and pieces I found see this discussion:
 
http://www.smartgroups.com/message/viewdiscussion.cfm?
gid=1423922&messageid=13593
 
Why at best?  Because the stomach acids destroy most T cells that reach
it by an oral route after the first 24 to 48 hours in ferrets, because
there are not likely to be many chicken and cattle diseases the maternal
cows and chickens had antibodies to that will also apply to ferrets,
because even IV therapy with Transfer Factor is short term and has
allergic risks with repetition, because given in a non-oral route there
are some medical problems for which can even be risky, etc.  Does it have
its uses?  There are some uses for IV (NOT ORAL) antibody therapy that I
found briefly mentioned but many of them were under study to see how
effective they are or if they are effective.
 
There are cheap placebos if a person wants something with that effect and
I really suspect that is all that this gives.
 
Also, remember that "boosting the immune system" isn't necessarily a
good thing.  What part of the immune system and how?  There are a number
of serious and damaging health effects of an immune system being more
active or wrongly active.
 
For example: the damage of ADV is due to the immune system leaving
plaques.  Fatal allergic reactions are another immune system overreaction
(in that case by the wrong part of the immune system).  Autoimmune
Anemia, which is typically fatal is just one of the autoimmune probelms
a ferret can get.  At least one of the vets currently studying the new
mystery disease/DIM thinks that an over response by the immune system is
a large part of the problem.  Cytokine Storms have been thought to be
responsible for deaths in many types of diseases in many types of
mammals, including for the disproportionately large number of young
people who died of the 1918 influenza epidemic.
[Posted in FML issue 4857]