A top U.S. health official is warning that 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.

Screening of blood donations in the island territory has indicated a rapid increase in the spread of the virus. Officials expect infections will increase through the summer.

Consequently, Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, said Friday that dozens or hundreds of births there could be affected.

More on this...

The island has more than 1,700 Zika cases, including 191 in pregnant women.

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