I'm sorry to hear this is happening to your hamster. There are a number of possible causes. If the bald areas are somewhat circular, this could be ringworm. If some of the sores are scabby, it could be a mite infestation or mange. Look for little black dots crawling on your hamster. If you see them, your hamster has mites. Mange is also caused by mites, but they aren't as visible. Ringworm, mange, and mites can all spread to humans and some other pets, so use caution in handling your hamster if you suspect one of these causes. Because your hamster has bald skin, it isn't safe to use over-the-counter mite sprays or other products on him. You would need a vet to treat any of these conditions. The following links will take you to directories of vets who treat small rodents. At the first site, you'll come to a log-in page. Sign in as 'member' and use 'member' as your password XXXXX well.
Another possibility is that the hamster is allergic to or being irritated by his litter/bedding. Sometimes animals itch so badly that they scratch themselves until they lose hair and create sores. If you're using cedar or pine bedding, get rid of it and try something else.
You didn't say how old your hamster is, but if he's an old hamster, it's not unusual for kidney failure to cause hair loss. Here's a link where you can read about all these causes of hair loss and skin problems:http://www.hamsterific.com/IllnessGuide.cfm#Ringworm
I would recommend that you take your hamster to a vet. Because there are so many different things that could be causing this, it would be very hard for you to figure out what it is. Other than changing the type of bedding, there isn't much you can do at home. A vet can also give you the best treatment for whatever it is he/she finds. This isn't an emergency, but I'd try to schedule an appointment next week.
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