In February, the World Health Organization’s declaration of a global health emergency confirmed what doctors in Brazil had suspected for months: The mosquito-borne Zika virus had exploded in the South American country. Since 2015, physicians had seen an unprecedented spike in babies born with abnormally small skulls and severe neurological deficits.
Babies born with microcephaly may not develop normally, leaving them severely disabled. Microcephaly is rare — occurring in 6 out of 10,000 live births in the U.S. — but suddenly there were reports of thousands of women in Brazil giving birth to babies with the birth defect, other severe fetal abnormalities or Guillain-Barre syndrome, a nervous system disorder. And even babies who appeared to be healthy developed serious deficits later.
Zika has been around for decades. The virus was first reported in Uganda in 1947, and there were a couple of outbreaks in the Pacific islands, including French Polynesia in 2013, but it was seen as relatively harmless. Public health officials didn’t realize Zika caused birth defects until they had a sizable outbreak in which clear patterns began to emerge.
“It’s conceivable that this was happening at low levels all along,” says Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in Maryland. “But it’s difficult to pick up these patterns in less-developed countries when there are so many other confounding variables.” Case in point: When epidemiologists looked back at data from the French Polynesian outbreak, they found an increased incidence of the same birth defects.
