There are alarming signs the Zika virus is spreading rapidly in Puerto Rico, the Centers for Disease Control and Prevention reported Friday.

Blood banks on the island have seen a steady rise in the portion of donations that have to be rejected because they contain Zika virus. Last week, 1.1 percent of the donated units were contaminated.

If that many people are infected with Zika when they go to give blood, it’s a sign of how much virus spread there is going on in Puerto Rico, CDC Director Dr. Thomas Frieden warned in an interview with STAT.

“What this means is that pregnant women in Puerto Rico are really at risk. That’s the bottom line here,” Frieden said.

“It isn’t that hundreds of thousands of people will die. That’s not what’s going to happen here,” he said. “It’s that we will have in Puerto Rico and potentially in parts of the US — in travelers and potentially even in some local transmission — we have terrible tragedies that will occur. Change people’s whole lives.”

Frieden said the blood bank data suggests that as many as 2 percent of adults in Puerto Rico are getting infected monthly at this point — and it’s not yet high season for transmission. Activity of mosquito-spread viruses typically peaks in the summer.

“If current trends continue, thousands of pregnant women will get infected with Zika,” said Frieden. “And there could be between dozens and hundreds of children with microcephaly born [there] in the next year.”

The CDC estimates 25 percent of the island’s population could be infected with Zika in its first year of spread there. Puerto Rican women give birth to about 32,000 babies a year.

Zika infection during pregnancy — particularly, it appears, in the first trimester — can lead to devastating birth defects.

The most notable — and the one that raised suspicions that the Zika virus could affect profound damage in developing fetuses — is microcephaly, a condition where babies are born with unusually small heads and brains that are not completely formed.

Over time, researchers in Brazil studying that country’s large cohort of Zika-affected newborns have reported other forms of brain damage, visual and hearing impairment, contorted limbs, and other birth defects, in what is coming to be known as congenital Zika syndrome.

Frieden noted it is still not clear if children who were infected in the womb but who appear physically normal at birth will experience development delays later. That has been seen with other viruses that can cause birth defects in babies born to women infected during pregnancy.

The primary focus of the CDC’s Zika response is to do all it can to reduce the risk that pregnant women will be infected. Frieden said the agency is working with local partners trying to identify effective steps that can be taken there.

HUD, the US government’s department of housing and urban development, is putting screens on the windows of public housing units, he noted. Many homes in Puerto Rico don’t have screens or air conditioning — features scientists believe will lessen the risk of wide spread of Zika virus in the continental US.

“We can’t make zero the number of infants who will be affected. But if we can reduce by 10 percent or 30 percent or 50 percent, we will have prevented that many tragedies. And that’s what we’re working to do,” Frieden said.

Health authorities in Puerto Rico have already reported one case of microcephaly there. No details were released but the pregnancy did not go full term.

Puerto Rico’s department of health reported Friday that 191 pregnant women on the island had tested positive for Zika. But a peculiarity of the disease the virus triggers means that number is almost certainly an underestimate.

The majority of people who contract Zika don’t have symptoms; it’s thought only 1 in 5 do. So it’s difficult to know how many people in affected areas have been infected.

Eventually scientists will do what’s called a sero-survey, taking blood samples from hundreds of people to look for the antibodies that indicate they were infected with Zika.

Studies done after the chikungunya virus — also spread by Aedes mosquitoes — swept through Puerto Rico showed that after the first year the virus was there, a quarter of the population of the island had been infected.

It’s too soon to do that work for Zika. Still, looking at the rate at which donated units of blood are testing positive provides a real-time hint of how many infections are taking place.

In fact, the estimate it provides is on the low side, Frieden noted, because anyone infected who has symptoms would be turned away if they tried to donate blood.

“It’s the closest thing we have to a real time pulse on what’s happening,” he said. “And what we’re finding is very concerning, because this is showing a steady and substantial increase in infections.”

This article is reproduced with permission from STAT. It was first published on June 17, 2016. Find the original story here.