When they first started seeing newborns with shrunken skulls last August, Dr. Vanessa van der Linden Mota and her mother, Dr. Ana van der Linden, didn’t realize they were looking at a looming public-health disaster.

But it didn’t take long for the two neuropediatricians to start connecting the dots. The tiny heads were classic signs of microcephaly, an incurable condition associated with incomplete brain development typically caused by chromosome disorders or maternal alcohol abuse. Unusually, though, some of the infants’ heads were draped with excess skin. Others’ skulls bore calcified patches that squeezed their brains in a vise grip. Some of their limbs were crumpled and bent at odd angles. Also oddly, in 70% of the cases the two doctors were seeing, mothers reported itching or rashes during their pregnancies.

Then there was this: In a typical year, the doctors said, they might see one microcephaly case every couple of months. Suddenly, they were seeing dozens.

By October, the two doctors alerted Pernambuco state health authorities and fingered what they believed was the likely culprit: the mosquito-borne Zika virus that is ravaging Brazil and sweeping rapidly across the Americas.

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“With all my years of experience, I never had seen an epidemic of this magnitude,” said Dr. Ana van der Linden, 75 years old, the matriarch of a family of five physicians.

At the time, the microcephaly connection struck some experts as implausible. After all, the Zika virus, discovered in Uganda in 1947, had long been considered a mild pest, rarely fatal, and certainly not associated with catastrophic birth defects. But now, the global medical community is starting to fear that the van der Lindens were right.

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