"The clusters of microcephaly and other neurological complications constitute an extraordinary event and a public health threat to other parts of the world," WHO chief Margaret Chan told reporters.
It now appears that a number of those cases of the microcephaly in Brazil may not in fact be microcephaly, or not linked to the Zika virus as authorities originally believed.
The World Health Organization warned Thursday that Zika is spreading "explosively" through the Americas, predicting up to four million cases. Costa Rica announced Friday that it had detected its second case.
"If you're not pregnant I don't think I would avoid going to Carnival or the Olympics for any reason whatsoever," one Zika virus expert said. The majority of people who get infected — estimated at more than 70 percent — experience no symptoms at all.
Airlines offered refunds to pregnant mothers afraid to travel to countries where Zika is present, while Costa Rica announced measures designed to keep the virus from getting a foothold in the country.
The first Zika patient in Costa Rica apparently contracted the virus while visiting Colombia. Health Ministry officials fumigated a 100-meter square area around the man's bedroom and interviewed neighbors, none of whom had Zika symptoms.
Some Latin American countries are warning women to avoid getting pregnant, after thousands of cases of birth defects linked to the disease in recent months.
The United States warned pregnant women Friday to avoid travel to 14 countries and territories in the Caribbean and Latin America due to the mosquito-borne Zika virus, which has been linked to birth defects.
