Officials are still looking at whether the virus causes birth defects.
(Bloomberg) — Zika virus, a mosquito-borne pathogen that may cause birth defects when pregnant women are infected, has been “spreading explosively” in South and Central America, the head of the World Health Organization (WHO) said Thursday.

“The level of alarm is extremely high,” WHO director general Margaret Chan said Thursday in an e-mailed statement.

Chan emphasized that researchers are still working to determine the exact link between the virus and birth defects. “The possible links, only recently suspected, have rapidly changed the risk profile of Zika, from a mild threat to one of alarming proportions,” Chan told members of the WHO executive board in Switzerland.

The virus, which has long been present in Africa and equatorial Asia, has since spread to 23 countries in the Americas, according to the WHO. While many populations where the virus has long been present have immunity, that’s not the case in newly infected areas, which has raised the agency’s level of concern.

Copyright 2018 Bloomberg. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.