Thor,
 
Asthma can be caused by ferrets, although, I imagine that, like everything
else, different people can be effected differently.
 
As for me, my roommate had ferrets in college.  I never had any allergy
problems then.  When my husband and I had our first apartment, we wanted a
pet that would be happy with just a room or 2 in which to run.  Ferrets
seemed like the perfect answer.
 
After we got our 2 babies (both fixed and de-scented, by the way), it became
obvious that I was pretty allergic to them.  My asthma got much worse than
it ever had been.  The doctor told me that the reason I didn't have a
problem with ferrets the first time is because you can't be allergic to
something that you have never been in contact with before.  So when I lived
with ferrets the second time, the allergies kicked in like gang busters.
 
The doctor would like me to give up pets.  As a child, being allergic to our
cat apparently contributed to my getting bronchitis and pneumonia 4 times in
one year (had to get rid of the cat that time, my mother made me).
 
But I've decided to take the risk.  I'm not willing to give up the fuzzies
(in fact ferret math hit a year after the first 2, and we got another).  So
I have a couple of prescription inhalers that I take every day, and I see
the doctor every 6 months or so about it.
 
The most effective things I have found are to change the ferret bedding (we
use old towels) twice a week, don't allow the ferrets (or the cat) into my
bedroom at all, and running an air filter/purifier in the bedroom.  (So the
bedroom is a "clean" room I can always retreat to if I'm having particular
trouble).
 
I also seem to do better when I keep the litter boxes and the ferret room
clean.  I've never been able to decide if washing the ferrets regularly
makes a big difference.  However, when we got the cat (who has more run of
the house than the fuzzies), the vet suggested wiping her down with a wet
wash cloth every morning.  I've been doing that for a year now and I have
remarkably little trouble with allergies to her, so I'm thinking that I
might try that with the ferts as well (but I shudder to think how much time
this will add to my morning ritual, I mean, once you trap each fert, give it
linotone, get kisses, hold it still and wipe it down with the cloth, dry it
off, give it some other treat, get kisses, give it a back scratch, give it a
wrestle, find a wind up toy for it, chase it around, etc)
 
I also have a small problem with eczema, which can flare up around the
animals and other allergens, but mostly that is aggravated by stress, (which
I hope your little brother doesn't have much of) so my eczema experiences
probably aren't of much use to you.
 
One thing I have noticed living a life with allergies is that an allergic
attack is rarely an isolated thing.  Its usually due to a combination of
things in my life.  If I'm having a problem with pollens, for example, then
I'll have even more trouble with the ferrets than usual.  Or if I eat a lot
of chicken or potatoes (yeah, isn't that pathetic?  I'm mildly allergic to 2
of the most common and tasty foods, not allergic enough to keep me away from
them, just enough that I know I shouldn't eat them, but I do it anyway, but
I digress) I'll need to stay further away from the animals for a few hours.
 
Anyway, sorry to babble on so.  These are just my experiences.  Your mileage
may vary.  I'm sure other people will respond as well.  Try what has worked
for us and see what happens.
 
Norris (with regards from Gunther, Nico, and Ozzie, and Mark, the hubbie,
and Miranda, the psycho-kitty)
[Posted in FML issue 1837]