Some ferrets (It is not common but we have had 2 in 21 years.) lose fur
in response to stress, usually physical stress, but Ashling (who lost
both adrenals cleanly many years ago) loses fur temporarily for emotional
stress (like when another ferret dies) and then regains it afterward.  On
possibility, therefore, is that the fur loss was from physical stress
from the fleas.  Also, they have been known to scratch fur off with
fleas, mechanically creating bald patches, and that is more common.
 
To read about reactions to fleas themselves, to Frontline or to
Advantage -- since there always will be some animals who react to
anything -- I suggest going to
http://listserv.cuny.edu/archives/ferret-search.html
or
http://fhl.sonic-weasel.org
because there is at least one past post from Dr. Murray which mentioned
such unusual reactions and what may be seen.
 
BTW, ferrets do not get rhinoviruses (colds).  Believe me, it's
been tried and tested because they'd be easier test subjects for
pharmaceuticals than some alternatives.  What they DO get are influenza
(which people sometimes confuse with colds if mild enough), bacterial
sinus infections (which people often -- really often -- confuse with
colds but which can become more serious for anyone two or four footed),
sometimes allergies, often sneezing to help release scent for rubbing
over their faces (Some of their glands are kind of below and behind ears;
sniff there after a sneeze and you'll notice that.), and also often
sneezing from mechanical irritation from inhaled dust.
[Posted in FML issue 4316]