Zika, which was first identified more than 50 years ago, has alarmed public health officials in recent months as it was potentially linked to thousands of cases of brain defects in newborns. Estimates are that the virus will infect up to 4 million people in the coming year.

The declaration represents the WHO’s highest level of alert and is only invoked in response to the most dire threats. The first time was in 2009 during the H1N1 influenza epidemic that is believed to have infected up to 200 million worldwide; the second in May 2014 when a paralyzing form of polio re-emerged in Pakistan and Syria; and the third in August 2014 with Ebola in West Africa.

Infectious disease experts and others have been pressuring the WHO to escalate its response to Zika for several months, warning of the mistakes world leaders made during the Ebola crisis when a lack of coordination delayed quarantines and treatment.

Margaret Chan, WHO director-general, said there is no reason for travel or trade restrictions at this time.

Much of the alarm comes from reports from Brazil, the epicenter of the outbreak, where Zika is suspected as a cause of what may be up to thousands of babies being born with abnormally small heads and incomplete brain development. Researchers are also investigating a possible link between the virus and a surge in Guillain-Barré syndrome, a rare condition that can lead to paralysis.

In an interview with the Reuters news agency, Brazilian Heath Minister Marcelo Castro said the outbreak is his country is worse than previously believed because an estimated 80 percent of people who become infected with the virus do not exhibit known symptoms.

Castro also said every municipality in Brazil will be required to report all Zika cases to a central database starting next week. In further controls, Brazil will join other nations in banning blood donations from people who had the virus.

Last week, Castro warned that Brazil was “badly losing” the battle against the mosquito blamed for spreading Zika and said that more than 220,000 members of Brazil’s military would be mobilized in eradication efforts. The plans included distributing mosquito repellent to about 400,000 pregnant women, according to Brazil’s O Globo newspaper.