The World Health Organization(W.H.O) director Margaret Chan declared the Zika virus and its suspected link to birth defects an international public health emergency on Monday, a rare move that signals the seriousness of the outbreak and gives countries new tools to fight it.

The  outbreak of the Zika virus, which is transmitted by mosquitoes, was detected in Brazil in May and has since moved into more than 20 countries in Latin America, including two new ones announced Monday: Costa Rica and Jamaica.

The Zika virus was first identified in 1947 in Uganda, and for years lived mostly in monkeys. But last May in Brazil, cases began increasing drastically. The W.H.O has estimated that four million people could be infected by the end of the year. The declaration will trigger funding for research to try to establish whether the Zika virus, spread by mosquitoes, is responsible for the large numbers of babies born with abnormally small heads and, in the vast majority of cases, damaged brains in Brazil.

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