If your hamster is doing anything out of the ordinary that worries you - coughing, wheezing, shaking, developing sores or callouses, walking weird, or any other abnormal behaviour - see a vet immediately because he might be very ill and will need immediate assistance.

Wet Tail:

Wet Tail is a bacterial infection or an imbalance of the natural bacteria in the hamster's stomach or intestine and is often confused with diarrhoea. Although a hamster with Wet Tail has diarrhoea, the diarrhoea is so severe that the hamster is wet and/or dirty not only around the anus but usually around the top of the tail as well. The diarrhoea is accompanied by a strong unpleasant smell. Droppings are pale in colour and extremely soft and may contain mucus. The hamster often walks hunched up, is weak and lethargic and may squeal in pain. Symptoms take 7 days to appear and the disease is often fatal with death occurring as early as 24 hours after the appearance of the symptoms. Therefore it is vital that veterinary treatment is sought immediately if a hamster shows symptoms of Wet Tail. Do not comletely rely on over-the-counter products such as Dry-Tail to treat Wet Tail as these are often not effective against severe cases of Wet Tail. These should only be used if it is not possible to get the hamster to a vet immediately and veterinary advice should still be sought at the earliest opportunity, but do ensure if you have used any over-the-counter product that you inform your vet when you visit.
Any equipment occupied or used by a hamster that has died from Wet Tail should be disinfected thoroughly with a disinfectant designed for small animal cages and left to stand for a few weeks before being used by another hamster.