Public health officials expect the prevalence of the virus to only increase in Puerto Rico in coming weeks and months. One big reason: The mosquito that most commonly transmits it, Aedes aegypti, is present throughout the island.

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“The risk to Puerto Rico is significant,” CDC Director Tom Frieden said recently.

The U.S. territory has experienced previous widespread outbreaks of dengue fever, another virus spread by the same type of mosquito.

Most of the people infected with Zika so far live on the the eastern side of the island or around the populous capital of San Juan, according to the CDC. Four patients have been hospitalized, but most have reported only minor symptoms, such as rash, joint pain or eye pain.

CDC officials said the first case there involved the elderly man whose symptoms -- diarrhea, abdominal pain, chills -- began in late November. He was hospitalized in intensive care and underwent a battery of tests before he was diagnosed, with officials finally confirming he had Zika in December.

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Another case in mid-January involved a 37-year-old man who developed a rash, followed by weakness in his upper and lower limbs, as well as his facial muscles. He eventually was diagnosed with a Zika infection as well as Guillain-Barré.

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Days later, a blood test confirmed a Zika infection in a woman in her first trimester of pregnancy. She had sought care after days of eye, body and joint pain, a rash and nausea. Officials said her doctor counseled her on the potential risks of microcephaly because of the virus and recommended close monitoring based on recent CDC guidelines issued for pregnant women.

Puerto Rico Gov. Alejandro Garcia Padilla last week declared a public health emergency because of the increase in Zika cases. The CDC on Friday urged Puerto Ricans, as they have urged residents in all Zika-affected areas, to take measures to avoid mosquito bites, such as wearing long-sleeved shirts and long pants, using insect repellent and making sure doors and windows have screens.

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Lena H. Sun contributed to this report.