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Yet with each passing day, he said, the association between the virus and microcephaly has become stronger. So, too, has a link between Zika and Guillain-Barre syndrome, which can lead to paralysis in adults. Several South American countries have identified cases of that rare condition.

Colombia’s health minister Alejandro Gaviria told reporters Friday that three patients with the syndrome died last week at a clinic in Medellin, the country’s second-largest city. All three were infected with Zika, Gaviria said, telling reporters that authorities believed the virus was to blame for their deaths. Two of the patients were from the town of Turbo on the country’s north coast; the third lived on the Colombian island of San Andres in the Caribbean.

In a report released Friday, the CDC said current information about possible sexual transmission is based on three cases. The first was “probable sexual transmission of Zika virus from a man to a woman, in which sexual contact occurred a few days before the man’s symptoms began.” The second is the Dallas case. The third is an instance of virus isolated from semen at least two weeks and possibly up to 10 weeks after illness began.

The virus remains in a person’s blood for about a week. But researchers don’t know how long it lingers in semen. “We know semen may have a large amount of viable virus for at least a short time after viral infection,” Frieden said.

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The agency is advising additional screening for pregnant women who live in areas with ongoing spread of Zika. Those with symptoms should be tested at the time of illness, CDC recommends.