There is always the concern with pets in Order Carnivora that allium
(garlic, onion, etc.) has the potential to cause a life threatening form
of anemia, hemolytic anemia, though cats are the ones at greatest risk
rather than dogs or ferrets.

http://ferrethealth.org/archive/browse.php?msg=SG14494
http://ferrethealth.org/archive/browse.php?msg=YG13850
http://ferrethealth.org/archive/browse.php?msg=YG4743
http://ferrethealth.org/archive/browse.php?msg=YG3737
http://ferrethealth.org/archive/browse.php?msg=YG2443

Dr. Karen Purcell has noted intestinal damage from garlic:
http://ferrethealth.org/archive/browse.php?msg=YG2412

Studies on garlic have not found it particularly useful for avoiding
parasites, but I do notice that we seem to have fewer problems with
mosquitoes when outside ourselves when we have had a lot of East Indian
food but that has many seasons which may deter those insects beyond
garlic (since we don't usually eat in ways that avoid allium content).

Ferrets are not particularly prone to worms and the post below goes into
the nuances as do others that can be easily found.

http://ferrethealth.org/archive/browse.php?msg=YG3737

In about 25 years with ferrets we have not had fleas and hope we remain
as fortunate.  We avoided ticks till Steve began walking about 3,000
miles a year, but they get in on his clothing they usually prefer me over
the ferrets, we have noticed.  Heartworm didn't used to be big in NJ but
recently it is much more common.

Do NOT give supplementary Brewers Yeast to any ferret with insulinoma.
The little bit in food is okay, but it lowers blood glucose mildly so
can pose a risk for any ferret with insulinoma.  It is *useful* for
ferrets with diabetes, not a huge help but a help never-the-less.
This, too, is covered in the archives.

-- Sukie (not a vet, and not speaking for any of the below in my
private posts)
Recommended health resources to help ferrets and the people who love
them:
Ferret Health List
http://www.smartgroups.com/groups/ferrethealth
FHL Archives
http://ferrethealth.org/archive/
AFIP Ferret Pathology
http://www.afip.org/ferrets/index.html
Miamiferrets
http://www.miamiferret.org/fhc/
International Ferret Congress Critical References
http://www.ferretcongress.org
[Posted in FML issue 5275]