The CDC reports on transmission of Zika virus in the USA. In the 50 states, all cases to date have been travelers who were infected elsewhere; no cases are locally acquired. This result is anticipated, but will possibly change this summer as populations of Aedes mosquitoes increase. Florida leads the nation was 82 confirmed cases followed by NY with 57. (Six have been reported in IN.) Among US Territories, Puerto Rico is a hot spot with 445 laboratory confirmed locally acquired cases (an underestimate as many cases go untreated). NY has a large Puerto Rican community and a large number of travelers to Puerto Rico. Texas has 27 confirmed traveler cases and is at greatest risk for locally acquired transmission due to climate and historical mosquito populations.
The rate of transmission (RO) of Zika where large populations of mosquito vectors exist, can be much greater than 1. Some estimates for Columbia, suggests rates can go as high as 3 to 6.6. Any rate greater than 1 can start an epidemic. Fortunately, the rate of transmission can be reduced by an informed population and effective mosquito control. Locally acquired cases of Chikungunya, (another virus that has recently established in the Caribbean) in the USA have been confirmed. Chikungunya is vectored by the same mosquito species as Zika which suggests the probability of some local transmission of Zika this summer is quite high. How high the rate goes depends of the effectiveness of prevention measures.