Brazilian health authorities are expressing grave concern and have declared a state of emergency, following the birth of 2,400 babies with small heads.
The babies have been born with a condition called microcephaly, which causes severe brain damage. Twenty-nine have died.
The culprit appears to be a virus carried by mosquitoes. Known as new West Nile virus, the disease was first identified in monkeys in Africa 70 years ago.
While causing mostly mild symptoms in adults, the disease can produce severe neurological damage in newborns.
The pathogen was reportedly found in the amniotic fluid in two women carrying babies with microcephaly.
The World Health Organization is monitoring the situation closely.
Cases began cropping up in Brazil in May. More than 150 Brazilian infants were born with microcephaly in 2014.
Scientists are blaming subtle effects of climate change for spreading the disease.
In recent years, West Nile virus has spread to the Pacific Ocean, where it infected 11,000 residents on the island of Yap. It sickened at least 28,000 people in Tahiti and West Polynesia.