Any ferret with a bad cough needs vet attention, even on an emergency
basis.  In fact, it is not a bad idea for a mild cough when it is one who
never coughs.  We've had one for whom we got an emergency appointment
when she had a cough.  The vet couldn't hear anything, but the cough was
very out of character.  A chest x-ray was done and she turned out to have
a bad case of pneumonia AND pleurisy together.  She was lucky and
survived.  Chest x-rays are extremely useful for finding problems with
them.  Cardiomyopathy, a terminal but treatable condition (for which
treatment gives both better quality and quantity of life) can also cause
coughing and the most usual form (and sometimes the less usual one, too)
show up on x-ray.
 
If you have had influenza or a sinus infection (not a cold) ferrets
can catch those.  Do NOT use any over the counter meds on your ferret
unless your vet says to do so, since a number of those can be fatal or
permanently damaging for a ferret, but do consider safely providing the
ferret with some steam to ease the respiratory tract.
 
Serious coughing is one of those things that it pays to jump fast on to
be safest, just like when blockage occurs, or there is an injury, or...
Otherwise it is possible to have an unnecessary death occur.
[Posted in FML issue 4074]