It’s no secret: Mosquitoes are out to get you. More than a million people worldwide die from mosquito-borne maladies annually, according to the American Mosquito Control Association.

And now those bloodsuckers have a new weapon in their arsenal: the heretofore rare Zika virus that is sweeping north in South America, NBC News reports.

Typically more or less harmless—causing, at worst, mild symptoms in one out of five people infected—health officials in Brazil now say the virus may be linked to a serious birth defect.

In that country, Vox reports, regions with the most cases of Zika also have had an increase in microcephaly, which prevents babies’ heads and brains from growing to full size.

"There very definitely is something significant going on with microcephaly," CDC viral specialist Ann Powers tells NBC. However, she adds, it is not clear if the increase in cases of the birth defect is connected to Zika.

According to the Pan American Health Organization, tests suggest the virus was in the amniotic fluid of two pregnant women. Ultrasounds in the same women show their babies may have microcephaly.

And the virus has been linked to at least seven deaths in Brazil as of Nov. 30, Vox reports. Discovered in 1947 in Uganda's Zika forest, the first notable outbreak of the virus didn’t occur until 2007 in Micronesia.

It arrived in the Americas last year, and has since been seen as far north as Mexico, according to Vox. Aedes aegypti, the mosquito that carries Zika, NBC reports, lives in much of central and south America, as well as south Florida and Texas, and Hawaii.

(At least scientists have developed malaria-proof mosquitoes.)

This article originally appeared on Newser: This Virus May Be Causing Birth Defects

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