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Hi. Welcome to Just Answer. My name is***** and I've been a veterinarian for 15 years. Thank you for your question. When a kitten is introduced from a humane society, the biggest things that may be at risk for the resident cats of the house are: Ear mites, fleas, intestinal parasites, FELV, FIV and upper respiratory infections. Pretty much the kittens at the human societies are treated for ear mites, fleas, and the intestinal parasites so those shouldn't be a real big concern. They are screened for FELV and FIV upon entering the shelters so they shouldn't be an issue either. Most likely the biggest issue from an infection aspect is if they bring an upper respiratory infection into the house, much like a kid bringing a cold home from school. Typically it isn't that big of a deal for an otherwise healthy, indoor cat of their ages. Probably the biggest issue is a behavioral one. Introducing the new kitten and the resident cats together to establish harmony. There are many good links / articles on how this can be done. The best suggestion I can give is to take things slowly as it can take a little time for everyone to figure out the new flow of things in the house.