The World Health Organization declared the Zika virus and its suspected link to birth defects an international public health emergency, a rare move that signals the seriousness of the outbreak and gives countries new tools to fight it.

An 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 main worry is over the virus’s possible link to microcephaly, a condition that causes babies to be born with unusually small heads and, in the vast majority of cases, damaged brains.

The current outbreak of Zika has taken the world by surprise. The 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 rapid spread is because people in the Americas have not developed immunity.

Brazil is due to host the Olympics in Rio de Janeiro in August, and President Dilma Rousseff’s chief of staff said there is no risk of cancelling the games due to Zika.