One of the most common ferret diseases is adrenal disease. Adrenal disease is tumors on the adrenal gland, which cause it to product too much sex hormone. Symptoms are hair loss, swollen vulva in females and sexual behavior in either sex. It cannot be treated effectively without the help of a vet. The most common treatments are a Lupron shot or surgery. New treatments are being developed, including a Deslorelan implant or injection. That seems to work even better than surgery, but does not work on all ferrets. Melatonin is a treatment that's been tried over the last 5-7 years, but it doesn't seem to have any lasting effect, and might actually mask signs of an adrenal tumor growing larger. Adrenal disease is either becoming more common, or perhaps being recognized earlier. Ferrets can get adrenal disease as young as 9 months (although that's uncommon). Most often you'll see adrenal disease around 3 yrs and older.

Another common ferret disease is insulinoma. Insulinoma is tumors on the pancreas that make it make too much insulin, thereby giving the ferret low blood sugar. This can be life-threatening. Treatment is either surgery to remove the tumors (they almost always seem to grow back, though), prednisone or prednisolone to help keep the blood sugar levels up, and sometimes a few other drugs that aren't commonly used. Adrenal disease and insulinoma seem to often occur at the same time, and sometimes when the adrenal disease is treated, the insulinoma gets a little better.

Another common problem is lymphoma, which is a type of cancer in the lymph system. Treatment for that is prednisone and sometimes chemotherapy. Lymphoma in a young ferret is usually much worse than if they get it when they're older.

As for prevention, adrenal disease is supposedly affected by artificial light cycles (like having the lights on at night), but that isn't proven. Insulinoma seems to occur more often when a ferret is fed sweet treats or too many carbohydrates. Many ferret foods contain grains (which are carbohydrates) and current thinking is that feeding a diet without (or nearly without) carbohydrates might help prevent insulinoma. There isn't much you can do that we know of to prevent lymphoma.

In general, feeding your ferret a good diet (high in protein and fat, low in carbs), giving it lots of time out of the cage and fun things to do, and having it checked by a vet once a year is your best bet on keeping your ferret healthy.

I could write a LOT more about the subject, but that's a quick overview.