If you join http://groups.yahoo.com/group/Ferret-Health-list you can ask
your question there.  It has a number of excellent ferret vets, a ferret
veterinary pathologist, and other experts so just pose your question.
 
In our home (20 years with ferrets in the family come June) I can not
recall any ferret getting a strain a second time around.  I have seen
people give ferrets influenza and ferrets give people influenza, and I
have seen that happen with a sinus infection which DID trade back and
forth multiple times in the same individuals.
[Posted in FML issue 3689]