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I have never heard of Guinea pigs eating one of their own. I used to raise them and kept a couple dozen together no matter what. They are born ready to care for themselves. They have hair, eyes open and teeth for eating anything the adults do. Yes, they nurse for a week or so.
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Yes you can get ringworm from a hamster. You can get rid of it on the hamster by dabbing a tiny bit of anti-fungal cream on it several times a day. Wash the spot before each re-application.