The condition is carried by symptomless parents and is thought to have arisen in Africa, the Caribbean and other areas as a protection against malaria.

But because it is symptomless and painless, glaucoma is often caught late, when partial to total vision loss is largely considered irreversible, doctors say.

Dr. Jamieson said some of the microcephalic births they are seeing are among women who were symptomless for the Zika virus, a troubling pattern.