With the discovery of Zika in malformed fetuses, Dr. Turchi’s team has been able to turn to the kind of task Dr. Heymann described. They have recruited about 1,000 pregnant women with Zika symptoms, and are following healthy and microcephalic newborns in the same areas. They work nights and weekends, eating sandwiches from the institute’s shop or meals of rice, beans and chicken provided by a research assistant’s mother.

There are now so many reported cases of microcephaly that a new problem has arisen: too many false alarms.

Anxious obstetricians across Brazil have reported babies who merely have small heads, or babies whose mothers had other problems, like severe alcoholism or family histories of malformations, conditions that should have excluded them from the research.

Brazil has already changed its definition of a small head, to 32 centimeters around from 33 centimeters, and may revise it again soon.

Dr. Turchi defends those decisions, saying a broad net had to be cast at first because so little was known.

“We didn’t want to get just the severe cases; we wanted to look at the broadest possible spectrum of the disease,” she said. “Then we can narrow it later.”

‘Perfect Epidemic Curve’

Loosed on a continent where no one is immune, Zika has the potential to infect tens of millions of people. It is now being transmitted in 33 countries with about 600 million inhabitants, the W.H.O. says. Health officials in Brazil are investigating thousands of reported cases of microcephaly that may be linked to the virus.