Precisely..every day undoubtedly thousands of people are returning from

areas where Zika is endemic. Given the the types of Mosquitos that serve as vectors for ZIka are widely present over much of the U.S. and the virus can be spread by sexual contact..it is the mobility of the people not the range of the mosquito that is the problem.
http://www.cdc.gov/zika/vector/range.html

I recall hearing about West Nile Virus a few years ago. I assumed it would be like Eastern and Western encephalitis, not very wide spread and more of a novelty in both horses and people. I was wrong. It was devastating in this area to a large population of horses and several human fatalities.
I would wager that if testing is done on a widespread basis where the Mosquitos are found, there will be many pockets of infection of Zika over the next couple years.