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]