A viral disease, known as ECE (epizootic catarrhal enteritis) or 'green slime' disease, causes severe, watery diarrhea in mature ferrets. Affected ferrets will become dehydrated in a few hours and require veterinary care. If you cannot immediately get to a veterinarian, get some PedialyteTM, an electrolyte solution made for human infants and sold in grocery stores. Give the ferret as much as you can get him to swallow, by scruffing him and gently dripping the fluid into the corner of his mouth with a plastic eye dropper. As a rough estimate, the ferret will need 10% of his body weight (measured in grams) in balanced electrolytes (measured in ml's) to replace fluid lost in watery diarrhea. An average jill weighs 700 to 800 grams, and needs 70 to 80 ml of electrolytes per day (28 ml = 1 oz). A male needs about twice as much.
Ferrets that are very dehydrated feel 'doughy': their skin does not slide along their bodies as it usually does, and if you pinch a fold of it, the fold will stay there when you let go. Their eyes are dull and often half closed. Ferrets in this condition need injectable electrolyte solutions very soon or they will die.
Ferrets with very soft but not watery stool might not require any treatment, depending on circumstances. If soft stool is caused by a dietary change, you can control the ferret's food intake until the problem resolves. If soft stool continues for more than a day, particularly if it contains mucous or blood, take the ferret to a veterinarian. Meanwhile, remove from the diet, foods that induce diarrhea, (e.g., dairy products). Cat laxatives given in large quantities can induce severe diarrhea and should also be withheld until the stool is normal. Drinking water should be provided in a dish to make sure the ferret gets all he needs. Make no drastic food changes until the stool returns to normal. To learn about other causes of diarrhea see: Digestive System, Liver and Pancreas.