"If you are a woman who is pregnant living in the U.S., there's one really important thing you need to know: You shouldn't go to a place that has Zika spreading."

That's the strongly worded advice from Dr. Tom Frieden, head of the Centers for Disease Control and Prevention in Atlanta. It's a marked contrast to the suggestion made by Dr. Margaret Chan, director of the World Health Organization, earlier this week: A pregnant woman could safely travel to affected countries, Chan said, if she wore long sleeves to ward off bugs and used mosquito repellent.

Frieden's statement is a sign of just how seriously the CDC is taking the Zika outbreak, which has been linked to a surge in birth defects in Brazil. CDC is one of the lead global agencies in the battle against this virus, and officials there say the outbreak is far from over, with major challenges ahead.

Indeed, Zika now dominates the agenda at the CDC's Emergency Operations Center. Giant video monitors in the front of the room track reports of Zika cases around the globe. A huge picture of the Aedes aegypti mosquito, which spreads Zika, sits next to a grainy photo of the virus taken through a microscope.

As you'd expect, the CDC is keeping tabs on Zika cases imported into the U.S. But it's doing far more than that, reaching out to help countries throughout Latin America test for the virus and attack the mosquitoes that carry it.

The CDC is working with researchers in Brazil to try to verify whether Zika truly is responsible for thousands of cases of microcephaly — a condition in which babies are born with abnormally small heads and possible brain damage.

Dr. Lyle Petersen, director of the division of vector borne disease and the head of the CDC's Zika response, worries that microcephaly might not be the only potentially serious health risk from what has typically been regarded as a mild virus. Zika may be causing other neurological problems as it spreads through Latin America, he says.

"So that's one of the things we want to find out," says Petersen. "We also want to figure out how we can best protect pregnant women from getting infected. That is the top priority."

The additional problems include cases of Guillain-Barre syndrome popping up in countries soon after the arrival of Zika. Guillain-Barre is a temporary but at times debilitating neurological disorder that can cause mild tingling and weakness — or that can paralyze. Researchers saw an increase in Guillain- Barre cases during earlier Zika outbreaks in Micronesia and French Polynesia.

The CDC's Leonard Peruski, with its global disease detection branch, sees a similar pattern: "a spike in Guillain-Barre cases most notably in El Salvador and Colombia. There have been some sporadic reports in Honduras."

In its latest figures, the Pan American Health Organization says that El Salvador reported 46 cases of Guillain-Barre in December. Two of the patients died. Guillain-Barre, which can have various causes, is fairly rare; the 46 cases in El Salvador are more than three times what that country normally reports monthly. A direct link, however, between Guillain-Barre and Zika has not been proved.

Until last week, Peruski was heading up the CDC's regional disease detection center in Guatemala. He watched as Zika spread across the region.

"It was much faster than we expected," he says. "We were doing some very simplistic models down there. We expected it to emerge probably this spring throughout Central America and we were surprised at how quickly it did spread. Again, this indicates that diseases don't respect borders."

The concern about this outbreak isn't limited to this hemisphere.

"This morning I was on a conference call with India," says Peruski. "They want to get testing set up. We have a request from Bangladesh. We have requests from Kenya, from China, from all over the globe, about how to get Zika diagnostics in place."

Besides testing, mosquito control is the other big issue CDC is trying to help with on the Zika outbreak.

The mosquitoes that spread Zika have been developing resistance to the most common insecticides. Researchers at the CDC's labs in Atlanta are studying patterns of resistance and trying to figure out how to overcome them.

"These are mosquitoes that are really difficult to control. They can breed in a bottle cap," says Frieden. That's all the water they need — just a capful. And that makes them a daunting foe.

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