Liz Szabo

USA TODAY

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.

Secretary of Health and Human Services Sylvia Mathews Burwell, who just returned from a two-day visit to Puerto Rico, said her department has awarded $5 million to 20 health centers in Puerto Rico. The money will help expand family planning services, including contraception, outreach and education.

"This problem demands our attention and our continued action," Burwell said Thursday at a press conference.

Puerto Rico is ill-equipped to fight the Zika outbreak or care for any babies born with microcephaly, a Zika-linked birth defect in which babies are born with small heads and incomplete brain development. The island faces a $72 billion debt. The Obama administration has proposed changing Medicaid rules for Puerto Rico to cover more people there.

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"We are obviously very concerned about the economy in Puerto Rico," Burwell said.

Obama asked Congress in February for nearly $1.9 billion in emergency Zika funding. Congress has not approved that request, and some Republican leaders criticized it as a "blank check" or "slush fund." Obama transferred $510 million in money formerly earmarked for Ebola for the Zika fight.

But Burwell said additional funds are urgently needed. Some of the money would go toward controlling the mosquitoes that spread the virus, developing a vaccine and caring for pregnant women and their children.

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Doctors will need to follow the children of women with Zika infections, even if they are not born with microcephaly, because it's possible they may suffer from less obvious types of brain damage or other neurological problems. In addition to microcephaly, some children with Zika have been born with eye problems that could impair their vision or even leave them blind.