Birth defects caused by viral infections can usually be sorted out based on the particular type of defect, indicating mostly the stage of fetal development at which the viral infection occurred. So if there was a particularly high rate of a particular defect in a particular area in a given period of time, this would mean that there was a viral exposure in the area. It could be mosquito borne, it could be transmitted through droplets, etc.
Zika, I believe, was first identified in Africa, but of course like all insect borne diseases, it immediately took ship (literally) for places in contact with Africa. A mosquito in the hold is enough to do it.
I travel a fair amount, and I know people get annoyed about the Dept. Of Agriculture agents prowling around the baggage carousel with their little beagles, sniffing for agricultural products. I’m not sure it does any good, and in any case, mosquitoes are not going to come in on my Spanish ham sandwich.
But at the same time, it’s important to do everything possible to keep these critters out - and exterminate them when they raise their ugly proboscis! I live in Florida, and of course tropical beasties love our climate, so there are a lot of pests, ranging from tiger mosquitoes and fire ants to pythons, that have managed to sneak in here.
Zika is blood borne.
It can also be passed as an STD, which makes its proliferation harder to control.