On Friday, the Centers for Disease Control and Prevention issued a travel alert for Mexico, Central America, South America, and the Caribbean, due to the outbreak of Zika virus in these regions and its association with birth defects. The alert is a level two (out of three), meaning the CDC advises travelers to “practice enhanced precautions”—in this case, protecting themselves from mosquito bites.
Zika is a mosquito-borne virus that is similar to dengue both in form (both are flaviviruses) and in symptoms, which can include rashes, fever, headaches, pain behind the eyes, and joint pain. However, while dengue can be so painful that it is sometimes called “breakbone fever,” Zika is “usually mild,” according to the CDC, and only one in five people infected will develop symptoms.
The urgency of this outbreak comes not from the severity of its symptoms, but from the fact that Zika has been linked to microcephaly—smaller-than-normal head size—in infants. This is typically a rare condition, and is associated with incomplete brain development. In Brazil, there were 20 times more microcephaly cases in 2015 than in 2014; the country saw its first case of Zika in May of last year.
The link between Zika and microcephaly was strengthened recently when the CDC found the virus in tissue samples from two Brazilian infants with microcephaly who died after birth, and two others who died in the womb, The New York Times reports. Some Brazilian doctors and health workers have been quoted in the media advising women in affected areas not to get pregnant, though it doesn’t seem to be an official recommendation.