seasonal fluctuations

I found a flea on our dog yesterday. She has never had any on her before. She is treated with Advantix for 7 months of the year. A visitor came in the house a few hours previous to finding the flea and he has a cat and had just been around someones dog. Is it possible the flea came in on him? We will continue to check the dog today but I am hoping this is an isolated incident. Your thoughts please.

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