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What you describe can actually be caused by several underlying diseases:
1) Fleas or a flea allergy - It is important that your cat is on flea prevention, as although you may not see fleas...there are times when 1 flea bite can cause a cat that has a hypersensitivity to be very allergic.
2) Another possibility is infectious causes such as "mange".
There are 2 different diseases that people use the term "mange" for: