WNV is actually nationwide, but this year’s outbreak is looking nastier than most. The biggest and most significant concentration in TX at the moment just happens to be Dallas.
WNV stretches from Key West, FL to Anchorage, AK, IIRC.
I understand WNV is widespread, but it still seems a bit odd this many being affected in just one county. Conditions must be good for these little nasties.
I think you’re right- I’ve read that it’s around wherever there are mosquitos.