James, ferrets are used for developing effective influenza vaccines.
There actually are and have been people on the FML who adopted such
ferrets afterwards, though I don't know if that is still going on.  (I
sure hope that it is.)  I don't know what other things they are used
for in relation to health (not colds because it was found that they
don't get those) but looking at Fox's _Biology and Diseases of the
Ferret_ may give you a range, ditto sites on zoonoses such as
http://research.ucsb.edu/connect/pro/disease.html#b7
 
You are right that the individuals used need to be healthy, with rare
exceptions.  I can recall a time many years ago when there was an offer
to adopt and care for ferrets with lympho from a research facility which
was studying the various forms; we were among those asked but Steve and
I chose to care for her ourselves at home.
 
Yes, ferrets have been used to improve health choices for ferrets.  Steve
and I were around from the beginning when the ferret community was first
trying to reduce the public tendency to freak about ferrets in relation
to rabies (a process that took about 10 years from start to finish with
breaks between some segments).  The first step for that was to find out
which rabies vaccine they tolerated.  That work actually was done at
Marshall Farms and largely done at their expense, with Dr. Judi Bell
involved.  When it turned out that one vaccine had bad health reactions
Judi adopted those few individuals, who thereafter lived normal pet lives
and one actually went on to live an unusually long life.  Then the next
step was that the USDA had to find out which ones worked in ferrets and
which of those worked for at least a year (They have not been tested
for over a year, but one worked for only 6 months and was dropped for
consideration for that reason.) Later the funding was gotten together
for the CDC to study two rabies aspects in one series of studies in
conjunction with the U. of Kansas and U. of Georgia.  The two things
under study were: how much do the common U.S. strains of rabies differ
in one species, and how safe are ferret in relation to rabies.  Those
particular studies did involve ferrets dying but within a few months of
their conclusion more ferrets were saved by changes to the Compendium of
Animal Rabies Prevention and Control than had been lost to the studies,
an effect that was greatly helped by the efforts to get that change
information out to state and local health officials and emergency room
personnel all over the nation, something which it makes sense for people
to continue doing using the info at
http://www.avma.org/pubhlth/rabcont.asp
(The Compendium of Animal Rabies Control)
http://www.cdc.gov/ncidod/dvrd/rabies/Ques&Ans/q&a.htm
(Easy to read rabies Q&A from the CDC)
 
For those curious about why shavings are inappropriate:
http://www.trifl.org/cedar.html
(epidemiological cedar and pine info)
[Posted in FML issue 4179]