ECE is not usually fatal, except in older ferrets, especially those with other diseases. ECE attacks the digestive system, and is thought to cause permanent damage. Young ferrets, under a year, may only have symptoms for a day or two. Older ferrets may have symptoms anywhere from a few weeks to a few months. In these cases, the ferret will need to be force fed in order to survive.
Duck soup is usually a good place to start.
The ferret may also need pedialyte, or even subcutaneous fluids, and an antibiotic for secondary infections. Once a ferret has been exposed to ECE, they can be carriers of the virus for up to a year without showing any symptoms. Insulinoma will eventually progress to death if not treated. Treatment consists of surgery to remove the tumors, which may or may not help, or medically with Prednisone and Proglycem, which will not cure the cancer, but will slow down the growth, and treat the symptoms. Insulinoma is the opposite of diabetes, which is extremely rare in ferrets, although not unheard of.