The U.S. Centers for Disease Control and Prevention on Monday added four more countries and territories to a growing list of places where travelers risk being infected with Zika, a rapidly spreading mosquito-borne virus.

The CDC added American Samoa, Costa Rica, Curacao, and Nicaragua to a list of 28 other regions, on the day The World Health Organization declared the virus - linked to thousands of suspected cases of birth defects in Brazil - an international public health emergency.

The CDC issued its first travel alert last month and has since been updating the list that cautions people, particularly pregnant women, from traveling to those areas. (http://1.usa.gov/1JuM6jt)

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The Zika virus is transmitted by the Aedes aegypti mosquito, which is also known to carry the dengue, yellow fever and Chikungunya viruses. There is no vaccine or treatment for the virus, which typically causes mild fevers and rashes. An estimated 80 percent of those infected suffer no symptoms whatsoever.

Brazil has reported nearly 4,000 suspected cases of microcephaly, in which infants are born with smaller-than-usual brains.