Zika Virus 'Spreading Explosively' in Americas, WHO Says The virus has been linked to the birth defect microcephaly.

 -- World Health Organization director general Dr. Margaret Chan said in an emergency meeting today that she was "deeply concerned" over the rapid rise of the Zika virus in the Americas. She called for an emergency committee to convene next week to discuss how to battle the disease.

Chan said the virus, which is linked to the birth defect microcephaly, is "spreading explosively." The birth defect causes babies to be born with abnormally small heads and can result in serious developmental delays and other lifelong effects.

WHO officials cited the lack of immunity in both people and mosquitoes as one likely reason the virus is spreading so rapidly in South America. Brazil, for example, is expected to have an estimated 1 million cases in less than a year.

"Last year, the virus was detected in the Americas, where it is now spreading explosively," Chan said today. "The level of alarm is extremely high."

At least 20 people have been confirmed to have the virus in the U.S., although all are believed to have contracted the virus while traveling abroad. WHO said the virus has been detected in 23 countries and territories in the Americas.

Health officials have been grappling with how to stop the virus, which is spread by the Aedes aegypti mosquito. Today, officials from the Pan American Health Organization said they expect 3 to 4 million people will be infected with the virus.

In many cases, symptoms of the Zika virus are mild and usually last no more than a week. Symptoms include fever, rash, joint pain and conjunctivitis, according to the Centers for Disease Control and Prevention. Approximately one in five people infected with the virus show symptoms.

A rare paralysis syndrome called Guillain-Barre has been seen in past Zika outbreaks and CDC health officials have started an investigation to determine if the two are linked. The syndrome is an immunological reaction that can also occur after other viral or bacterial infections.

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