they don't need to be brought in by a pet, they'll just fucking appear
They ride in on people all the time - going to pet stores that let customers bring their dogs in is a good way to pick them up, but just being outside is enough.
You say you have indoor cats, though. I've had a couple flea infestations, and I've usually found they prefer the cat - until the cat is at the vet overnight they generally left us alone. Neither of us are allergic, though. If your wife is, she'll notice bites most people (including yourself) would miss.
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They ride in on people all the time - going to pet stores that let customers bring their dogs in is a good way to pick them up, but just being outside is enough.
You say you have indoor cats, though. I've had a couple flea infestations, and I've usually found they prefer the cat - until the cat is at the vet overnight they generally left us alone. Neither of us are allergic, though. If your wife is, she'll notice bites most people (including yourself) would miss.