Lack of playfulness, loss of appetite, dull/glassy eyes, etc. Can be symptomatic of a number of problems
Lack of Bowel Movement
If your ferret has gone longer than usual without using the litter pan (or some other corner) productively, he may have an intestinal blockage. Certainly by the time it's been 24 hours you should go to the vet immediately. Note that a ferret can continue to defecate for as much as a day even with a blockage, since there's still waste in the intestines to be eliminated. More often than not, [the cause of a lack of bowel movement] is a lack of food intake for some other reason. Ferrets generally go to the litter three or four times a day. Owners should look for adequate stools, although some may be a little loose. Also look for string-like stools. Ferrets with intestinal blockages can continue to pass stool which is very thin- like a pencil lead. But adequate ferret-proofing is much more important than stool-watching.
Swollen or Painful Abdomen
Bloating may come from many problems such as heart disease, splenic enlargement, or even just fat animals. Pain could be from any of several disorders, but the most common is an intestinal blockage, caused by eating something indigestible such as a sponge or an eraser. Not all blockages cause abdominal pain, though.
Change In Bathroom Habits
Suddenly refusing to use a litter pan or missing a lot more than usual, signs of discomfort or distress while using a pan, or any funny color or texture in the feces or urine could be a sign of any of a number of problems. Stress, perhaps from a change in environment, can also cause this.
Lumps On The Body Or Feet
These may be cysts or infections, or they might be associated with a tumor, usually benign but sometimes malignant. They can also be a sign of dietary problems or a vaccine reaction. Have any swelling or lump checked out and probably removed by your vet, and have anything that's removed sent to a pathologist.
Difficulty Using The Hind Feet, Awkward Gait, Lack Of Movement
Most often a sign of an adrenal or islet cell tumor (insulinoma), or arthritis, in older ferrets. Could also be an injured back, the result of having been stepped or sat upon, closed in a door, or the like. Ferrets have very flexible spines, but they're easily injured. This is a common finding in older animals of many species - the most common cause is a mild degeneration of the nerves in the spinal cord or those innervating the legs. In most of these cases, there is nothing to be done, but it also rarely results in paralysis, just variable amounts of weakness.
Ferrets do not tolerate high temperatures well at all. They (like any pet) should NEVER be left in a hot car, and if you're keeping them outdoors be sure to provide some shade and plenty of water in summer. Allowing them to sleep under hot radiators is probably also a bit risky. Temperatures as low as the 80's can be life-threatening to ferrets without shade and cool water.
Loose Skin And Dry Eyes
Generally caused by dehydration, which is quite serious in such a small animal. Get your ferret to drink more, take him to a vet for subcutaneous fluids, and look for the underlying cause
Unexplained Hair Loss
Not the usual seasonal shedding, which should happen twice a year (but the times may vary due to indoor lighting conditions), but a severe loss, especially if more than the tail is affected.
It's pretty obvious that these indicate some kind of problem. Most often the result of insulinomas in the pancreas causing extremely low blood sugar, but there are many other causes too.
This can be serious, since ferrets are easily dehydrated. Diarrhea may be caused by milk products, which contain lactose that ferrets do not tolerate well, or by a number of diseases. A green or orange color or a bit of mucus just means the food didn't spend the usual amount of time in the digestive system, not that it's necessarily ECE (Epizootic Catarrhal Enteritis, or the "Green Diarrhea Virus"), One thing you can try for mild cases, especially after consulting your veterinarian, is Pepto Bismol. Most ferrets don't like the taste of the liquid, but you can give them 1/15th of a tablet crushed up in food instead. A compounding pharmacist can also prepare the medication in Pepto Bismol in a different suspension to minimize or mask the taste.
Ferrets do sometimes vomit from excitement, stress, a change of diet, or overeating, but if it's repetitive or if there are any signs of blood, get to a vet. During shedding season ferrets may "spit up" a bit due to hair in the throat. This can be helped with Petromalt.
Sneezing, Runny Nose, Watery Eyes
Yes, ferrets catch human flu. They'll generally rest and drink a lot. A visit to the vet would probably be a good idea, particularly if the flu looks bad or lasts more than a few days. "The antihistamine product Chlor-Trimeton may be used at 1/4 tablet 2 times daily for sneezing that may interfere with sleeping or eating."
If only the tip is broken, the tooth may discolor slightly, but it's nothing to worry about. A more extensive break will cause pain, a definite unhealthy look to the tooth, and possibly gum problems, and should be treated (probably root canal or removal) by a vet or a veterinary dentist.
Persistent Hacking Or Coughing
An occasional cough might be caused by dust or swallowed fur, and can be treated with a bit of cat hairball preventative. A cough from a cold can be treated with children's cough medicine; ask your vet for a recommendation and dosage. A persistent cough is most likely a respiratory infection, probably viral. A fever, yellow or green discharge from the eyes or nose, or congestion indicate a bacterial infection. In either case, see a vet. Another possibility is cardiomyopathy.
In an unspayed female, she's probably going into heat, especially if it's springtime. For young spayed ferrets, under 18 months or so, the most common problem is pieces of the ovary that were missed in the spaying and have begun to produce hormones. These pieces might be scattered around the abdomen. For older ferrets, however, by far the most common cause of a swollen vulva is adrenal disease, usually cancer
Return To Whole Male Behavior (in a neutered male.)
The most common reason for a neutered male to try to mate, dribble urine or mark his areas, become aggressive, or have erections is unusual hormone production caused by adrenal disease. Other possibilities include cryptorchidism (a testicle which never descended into the scrotum and so wasn't removed) or bladder stones. The treatment for any of these is surgery.