"Researchers here tested the spinal fluid of 12 babies with microcephaly, all of whom were born to mothers who reported having symptoms of Zika early in their pregnancies. In all 12 cases, the researchers found evidence of Zika — results they described as stunning." Did they test any controls? If Zika is highly prevalent in the area where the babies were born, how do they know that it's not normal for all babies to have "evidence of Zika"? It is suggestive (but doesn't prove) a causal link if the number of cases observed to have a particular exposure (in this case Zika) is greater than the number you'd expect - but we don't know what the expected rate would be. Or do we?