“The level of concern is high, as is the level of uncertainty,” said Margaret Chan, Director-General of the World Health Organization, at a briefing. “Questions abound. We need to get some answers quickly.”

The virus, which can lead to severe birth defects, has led to a slew of emergency measures in countries across the Americas. El Salvador called on women to delay having children for two years. Brazil has sent more than 200,000 troops to eradicate the mosquitoes that carry the disease. The U.S. officials have said the country is launching a “full-court press” to address the issue.

WHO officials will meet on Monday to decide whether the disease outbreak qualifies as an international public health emergency.