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.

The CDC’s alert emphasizes that more research is still needed to determine if Zika is actually causing microcephaly. But “until more is known, and out of an abundance of caution,” the agency does recommend that pregnant women consider delaying travel to the areas where Zika is spreading, and that women who are trying to get pregnant talk to their doctors before going.

Zika is named for the Zika forest in Uganda, where it was discovered in 1947, but it has since traveled, causing outbreaks in other African countries, Southeast Asia, the Pacific Islands, and South America. In December, it hit Puerto Rico, and a case of Zika was confirmed in Houston, Texas last week, in someone who had recently traveled to Latin America. Other than Puerto Rico, it has yet to infect anyone within the United States—the only patients so far have caught the virus elsewhere and brought it back with them—but there has been transmission in 14 countries in North and South America, and “further spread to other countries in the region is likely,” the CDC writes in its advisory.

In a piece for The New England Journal of Medicine, Anthony Fauci and David Morens of the National Institute for Allergy and Infectious Diseases note that Zika is the fourth mosquito-borne illness to creep across the Western Hemisphere over the past 20 years, following dengue, West Nile, and most recently, chikungunya. As with these other diseases, there is no currently available vaccine or treatment for Zika—you just have to wait it out.

There may be a pattern here, in which relatively unknown, uncommon viruses that were fairly geographically restricted suddenly spread, far and fast. “Zika is still a pandemic in progress,” they write, “yet it has already reinforced one important lesson: In our human-dominated world, urban crowding, constant international travel, and other human behaviors combined with human-caused microperturbations in ecologic balance can cause innumerable slumbering infectious agents to emerge unexpectedly.”