cat hissing and viting

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Has anyone ever had a perfectly wonderful loving cat all of a sudden start to hiss and sometimes bite when someone unfamiliar comes into the hoouse? We have had our cat for 11 years, and in the last year she has started to become real ugly sometimes when we have guests in. Is this simply a sign of her aging, or does anyone have any ideas of what we can do about this? We love her and treat her as a member of the family, She is very tolerant with the grandkids, but all of a sudden does not want any strangers on her turf. Any and all help appreciated!