Nowhere in the U.S. has been hit harder by the Zika outbreak than Puerto Rico, where 570 people have been diagnosed with the virus, including 48 pregnant women. The true number of Zika cases in Puerto Rico, where the virus is spreading among local mosquitoes, could be much greater. Only about one in five people with Zika develop symptoms, so most of those with the virus are unaware they’ve been infected, according to the Centers for Disease Control and Prevention.

Although 426 travelers in the continental U.S. have been diagnosed with Zika after visiting an outbreak zone, the virus is not yet spreading among local mosquitoes on the mainland. Doctors worry most about the dozens of pregnant women in Puerto Rico who have been infected with Zika, which causes catastrophic birth defects and has been linked to rare cases of paralysis. FULL REPORT