Sporadic dengue fever outbreaks in Florida in 2009 and 2010 spurred mosquito control efforts in Key West and Miami Beach, shown here. The same mosquito that carries dengue, Aedes aegypti,can transmit Zika.
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A patient suffering from Guillain-Barre syndrome recovers in a hospital ward in San Salvador on Jan. 27. Researchers are trying to determine whether there is a link between the disorder, which can cause weakness and paralysis, and the mosquito-borne Zika virus.
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Dr. Margaret Chan is director-general of the World Health Organization. In her first major address on Zika, delivered Thursday in Geneva, she said: "Questions abound. We need to get some answers quickly."