This means that medical scientists have to look carefully at the epidemiology of the cases, and search for neurological (especially foetal) damage in previous outbreaks. They will look at the genome of ZikaVirus (ZIKV) to see if it has mutated, and if so, where (geographically) it mutated, and why. The answer to the last question is most likely "It was a spontaneous mutation", but causes should nevertheless be looked for. There are about 20 main causal agents for microcephaly.
Cytomegalovirus and Rubella virus are known to be able to cause microcephaly, and so ZIKV would be the third virus on the list.
Establishing the causal link beyond reasonable doubt could take years, though we will probably have a firmer idea about the cause of the microcephaly in a few weeks. The thing is, there is loose, sloppy thought behind the question "Can you prove that ZIKV causes microcephaly?" Science, unlike mathematics, does not prove things.
But the key thing is that community-based Mosquito Habitat Source Reduction - clearing stagnant water etc- must go on.
It must go on for one very good reason: even if (against all expectations) it should turn out that ZIKV does not cause microcephaly, it is pretty clear that it causes Guillain-Barre syndrome. It is also true that mosquitoes carry malaria and about 15 other diseases that cause illness, death, poverty, inequality and generally hold humanity back. On the other hand, a victory over the mosquito would be a huge boost to community morale.
David Heymann said another interesting thing. We do not know for certain that ZIKV could not gain a foothold in a British based mozzies. This means that we in the UK need to keep calm and keep a firm grip on possible mosquito habitats.