NEW YORK — Dozens or hundreds of babies in Puerto Rico could develop severe birth defects because of Zika, based on how an outbreak is playing out there, a top US health official said Friday.

The island territory has been screening blood donations for the virus since April. Last month’s results suggest there’s been a rapid increase in infections, and officials expect cases to increase through the summer.

‘‘In the coming months, it’s possible that thousands of pregnant women in Puerto Rico could become infected,’’ said Dr. Tom Frieden, director of the US Centers for Disease Control and Prevention.

According to a CDC calculation, that could lead to dozens or hundreds of infants being born in the coming years with microcephaly, a severe birth defect in which a baby’s skull is much smaller than expected because the brain hasn’t developed properly. Other infants may develop other, more subtle birth defects as well, Frieden said.

The Zika virus — spread mainly by a tropical mosquito — causes only a mild illness, at worst, in most people. But infection during pregnancy can cause fetal deaths and potentially devastating birth defects.

Puerto Rico is part of a Zika epidemic that has been sweeping through Latin America and the Caribbean.

Associated Press