The first baby born in the state with a severe Zika-related birth defect was reported by New York City health officials on Friday.

Doctors diagnosed microcephaly, a condition marked by brain impairment and smaller-than-normal head size.

Neither the hospital where the birth occurred nor the parents were identified by New York City Health Department officials. They said the mother had traveled to a region of the world where the viral infection is flourishing and returned home to New York infected. The baby’s gender was not disclosed.

“The City has been preparing for this scenario for many months now,” Dr. Herminia Palacio, deputy mayor for health and human services, said in a statement. “This case is a sad reminder that Zika can have tragic consequences for pregnant women.”

The birth of an infant with microcephaly served as a stark reminder of what little is known about a pathogen that only recently emerged in this hemisphere.

“I remind all pregnant women in New York City, and those trying to get pregnant, that they should delay travel to places where there is active Zika transmission,” said Dr. Mary T. Bassett, city health commissioner.

Bassett said city health officials “are monitoring the baby’s health closely and connecting the family with the necessary services to take care of their child.”

The new case is one of more than 1,500 babies born globally with microcephaly and other birth defects associated with the virus, estimates from the World Health Organization show.

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A majority of the Zika-related microcephaly cases have been diagnosed in South America, mostly Brazil. Outside of the microcephaly birth reported Friday, 12 other infants have been born in the United States with Zika-related birth defects, according to the Centers for Disease Control and Prevention. The infection can be transmitted by mosquitoes or sexually, experts say.

Zika virus belongs to the vast and complex pathogenic family known as flaviviruses, which include the yellow fever and West Nile infectious agents.

Dr. David Garry, a professor of obstetrics and gynecology at Stony Brook University School of Medicine, said three of six babies whose mothers had laboratory-confirmed Zika infections have been born at Stony Brook University Hospital healthy and free of Zika complications. Three other women, who are still pregnant, are awaiting the births of their children.

Garry said medical science has yet to explain why some babies of infected mothers develop brain abnormalities while others are born unscathed.

“Is it something about the viral load?” Garry said about the amount of virus received during the infection. “Is it something about other viruses, other flaviviruses that you may have had in the past? These are questions that nobody has an answer for.”

Dr. Jill Rabin, co-chief of ambulatory care and women’s health programs at Northwell Health in New Hyde Park, said questions abound. “We are not exactly certain how, but we know that the virus can cross the placenta and that it attacks the fetal nerve cells including the cells that develop into the brain,” she said.

Cells specifically affected include those that form the brain’s early scaffolding. Outside of Zika, Rabin said, other infectious agents, such as the Toxoplasma gondii parasite, can cause brain malformations.

In Pittsburgh, microbiologist Carolyn Coyne is studying how the virus crosses the placenta to cause fetal harm, but to date has no definitive answers.

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“What makes Zika unique is that we are talking about two people, the mother who has her own immune system, and the fetus who is related, but not identically,” said Coyne of the University of Pittsburgh.

She added that Zika is not the only virus that causes birth defects. Herpesvirus and rubella also cause birth defects. No one knows how either causes abnormalities in the developing fetus, Coyne said.