It certainly does seem strange that these birth defects haven't been seen in other countries with Zika.
I would suspect something else is going on in Brazil. But if I were pregnant, I wouldn't travel to a country with a high infection risk; because it is just too much of an unknown. I feel very sorry for anyone there trying to have a family, it must be terrifying just not knowing (not to mention something is causing this!)
My MIL however doesn't even want us coming to Texas (they have ONE non-locally transmitted case in her county). That's ridiculous. It's not that difficult to avoid mosquito born illnesses in the USA, since most people spend most of their time in air conditioned buildings.
When they went back to look at previous, small outbreaks of Zika in some Pacific islands, they did find an uptick in birth defects. At the time, the connection wasn't made because it was not clear if it was just a freak uptick with other causes. For example, if there were an island with 10,000 people on it, and normal rate of this birth defect was even as high as 1%, it would still be tough to say if 5 cases of a birth defect in a year was just random (esp since microcephaly can be caused by other things).
It's also very difficult to prove causation, without purposely infecting a population, which is why the CDC and others have not said for certain. However, there is mounting evidence that the huge uptick in microcephaly is caused by Zika. There's also evidence that it can be passed via sexual intercourse.
All that said, Zika is an order of magnitude less concerning than Ebola, though far more transmissible. If Ebola ever airborne (which would be an evolutionary feat, although not totally impossible) we'd be fucked. The number one worry among epidemiologists is probably plain old iinfluenza. Super-prone to mutating, passes easily between species (especially birds), some strains are *highly* virulent and it's super-transmissible. Increasing poultry markets, climate change, political issues all have the potential to really spur on prime conditions, but it's still generally a law of odds. If it mutates enough times, one of those will be deadly to humans (as we have seen a few times in history). We only have more people, and more connection with those people, than we did in 1917, for example.