That's another excellent question.
The Zika virus originated in Africa and in Asia. As you mentioned it was first detected there in 1947, but it was not in the Americas. This virus is a slightly different strain. I believe the genetic similarity is about 99% or 98%.
In the Americas you have people who have never been infected before so there is no immunity. In Africa and Asia people have been infected on an ongoing basis, don't know they're infected, and develop an immunity, but we have a naive—that's the technical term—population in the Americas who have never been infected.
The theory or some of the theories we have is that an infected individual—French Polynesia I think was one of the first places—came to the Americas, potentially Brazil, and got bitten by a mosquito and then infected somebody else and then it just blew up. Literally, millions and millions of people are infected in the Americas right now.
Why it blew up so quickly probably has to do with the slight variation. We don't know if there's an animal reservoir of some sort, a lot of viruses like Ebola have animal reservoirs. However, at this point the theories are that it's a naive population and there's no inherent immunity. That's why it's spreading so quickly.