Once again, I'm not a vet or even a ferret expert, but here's a list
of several of the most common medical problems in ferrets.

Intestinal blockages
   Caused by eating something indigestible, such as an eraser, a
   rubber band, some fabrics, or even a good-sized hairball
   (accumulated from grooming), which gets stuck.  Symptoms may
   include (one or more of) lack of bowel movement, constipation,
   bloating, vomiting or heaving, drooling, and others.  Blockages may
   occur at any point in the digestive tract, from the throat through
   the lower intestine, even in the stomach where the object may move
   around and produce only intermittent symptoms.  Blockages are
   serious and occasionally fatal; the most important immediate
   concern is to keep your ferret hydrated, which you can do by giving
   him 5 cc of water every 4 hours from a baby feeding syringe.  You
   can try giving your ferret large doses of hairball remedy every 30
   minutes for an hour or two to see if the blockage passes, but if
   not, take him to a vet right away for an X-ray, barium study,
   and/or surgery to remove it.  Laxatone or a similar hairball
   remedy/laxative can help prevent this [6.2].  

Tumors or lesions of the adrenal glands
   Symptoms vary, including hair loss spreading from the base of the 
   tail forward [10.6], lethargy, loss of appetite, and loss of
   coordination in the hindquarters.  In females, often the most
   prominent sign is an enlarged vulva as in heat.  Often, however, a
   tumor will be present without showing any signs at all, so if your
   ferret is going in for any surgery, the vet should take a look at
   the adrenal glands as well (if time permits -- ferrets lose body
   heat very quickly in surgery).  The left gland seems to be affected
   more often than the right.  More information is available in the
   Ferret Medical FAQ on Adrenal Disease [1.1].

Islet cell tumors (insulinoma)
   These are tumors of insulin-secreting cells in the pancreas.  Their
   main effect is a drop in the blood sugar level, and they are also
   common enough in older ferrets, even without symptoms, that if your
   pet is having surgery for something else, a quick check is
   worthwhile.  Symptoms include lethargy, loss of appetite, wobbly
   gait, and pawing at the mouth; in more severe cases attention
   lapses (staring into space) or seizures may also occur.  If you're
   more than a minute from your vet and your ferret has a low enough
   blood sugar level to be having seizures, call the vet and ask if
   you should rub Karo (corn sugar) syrup or honey on your pet's gums
   to raise it just enough to bring him out of the seizure.  More
   information is available in the Ferret Medical FAQ on Insulinoma 
   [1.1]. 

Lymphoma or lymphosarcoma
   This is a cancer of the lymphatic system.  There are two main
   types, "classic" and juvenile.  Classic lymphoma occurs in older
   ferrets and causes enlarged lymph nodes and irregularities in the
   blood cell count, but often the ferret doesn't show any outward
   signs until the disease has progressed pretty far, at which point
   the ferret suddenly gets very sick.  Conclusive diagnosis is by
   aspiration or biopsy of a lymph node, and treatment is
   chemotherapy.  Juvenile lymphoma is completely different. It
   affects ferrets under 14 months, doesn't generally cause
   enlarged lymph nodes, and hits very hard and fast.  Also see
   the Ferret Medical FAQ on Lymphosarcoma [1.1].

Splenomegaly [enlarged spleen, usually a swelling in the upper abdomen]
   In situations where a neoplasm is not present [this is a common
   symptom of lymphosarcoma], the pros and cons of splenectomy should
   be discussed with your veterinarian.  If an animal simply has a
   large spleen, but shows no signs of illness or discomfort, it is
   safer for the animal to leave it in.  However, if the animal shows
   signs of discomfort, such as lethargy and a poor appetite, or a
   decrease in activity, the spleen should probably be removed.  These 
   animals also need good nursing care [12.6] care to get them back on 
   their food.  Often caused by H. mustelae infection (see below).  
   With proper care - recovery rates are over 90%.  Also see the 
   Ferret Medical FAQ on Splenomegaly [1.1].

Helicobacter mustelae infection
   A bacterial infection of the stomach lining, Helicobacter
   mustelae is extremely common in ferrets.  Animals with
   long-standing infections (generally older animals), may develop
   gastric problems due to the bacteria's ability to decrease acid
   production in the stomach.  Signs of a problem include repetitive
   vomiting, lack of appetite, and signs of gastric ulcers (see
   above).  Helicobacter infection and gastric ulcers often go hand in
   hand - the relationship between infection and gastric ulcer
   formation has not been totally worked out, although there is
   currently a lot of research in this area.  Also see the 
   Ferret Medical FAQ on Gastric Ulcers / Helicobacter mustelae [1.1].

Cutaneous vaccine reactions
   Subcutaneous vaccination with rabies or other vaccines may, over 
   a period of weeks, cause a hard lump [10.1] at the site of 
   vaccination.  The lump simply consists of a large area of
   inflammation and most commonly are seen around the neck.  The lumps
   can be removed, and generally do not cause a major problem for your
   pet.  Similar lesions may be seen in vaccinated dogs and cats.

Urinary tract infections and prostate trouble
   Signs include frequent urination, straining to urinate, and
   possibly funny-looking or smelly urine.  Un-spayed females in heat,
   and spayed females with swollen vulvas due to adrenal disease 
   [1.1], are particularly prone to UTIs.  Treatment generally 
   consists of a course of antibiotic (usually Amoxicillin); if the
   ferret doesn't respond to that, the possibility of bladder stones
   [11.2.1] should be considered.  

   In males, what looks like a UTI may be (or be aggravated by) an
   inflamed prostate, also generally caused by adrenal disease.  In 
   this case the prostate, which is normally tiny, can be palpated,
   and a greenish goo can often be expressed from it.  Taking care of
   the adrenal problem should clear up the prostate trouble too.