Until 2014, Brazil had no more than 200 cases of microcephaly, a debilitating neurological disorder where newborns have an abnormally small brain. In 2015, the country recorded nearly 3,000 cases. Some of the worst affected areas have declared a state of emergency.

Many born with microcephaly die young. Those who survive have life-long cognitive impairment. To understand the sudden rise, in November, the country’s health ministry drew a link to an epidemic of Zika virus that began in early 2015.

Zika virus is transmitted by mosquitoes, and it was first detected in Uganda in the 1940s. After spreading through Africa and parts of Asia, it has made its way to Latin America. There is no known vaccine to prevent or medicine to treat the disease caused by the virus.

Since May 2015, the Brazilian government estimates that some 1.5 million people have been infected with the virus. In children and adults, the infection is mostly benign: some suffer from fever and red rashes, while others may be symptomless.

However, after finding the virus in the placenta of children born with microcephaly, Brazilian doctors have been warning women to delay their pregnancy if at all possible. “Most” mothers of microcephalic children, according to CNN, had Zika-like symptoms early in their pregnancy.

There is no known physiological basis for how Zika virus can cause microcephaly, and previous epidemics do not help make the case. A 2007 outbreak on Yap Islands in Micronesia is estimated to have affected nearly 75% of the population of some 12,000 people, and a 2013 outbreak in French Polynesia affected nearly 28,000 of 270,000 residents. Neither epidemics caused a spike in microcephaly.

An explanation for the link may be that a new strain of the virus is spreading through Brazil, according to Alain Kohl, a virologist at the University of Glasgow who studies Zika. Still, even for the fastest evolving organism on the planet, acquiring completely new powers of devastation is rare.

A more likely explanation is that the link has simply gone unnoticed so far. It may be that Zika-induced microcephaly occurs only in a small proportion of pregnant women, and none of the previous epidemics have affected a large enough population to raise an alarm.

Adding to the woes

Before the Zika epidemic began, Brazil was already dealing with a dengue epidemic spread by the same mosquito (Aedes aegypti) that is responsible for spreading the Zika virus. In 2015, the country recorded 1.6 million cases of dengue, nearly three times as many as that in the previous year.

“Brazil offers the ideal conditions for Zika to spread so quickly,” Ana Maria Bispo de Filippis, a leader of the research team that has linked Zika to microcephaly, told the New York Times. “It has a susceptible population in which the majority of people never had contact with the disease.”

The country’s political situation isn’t helping. The president, Dilma Rousseff, is fighting impeachment proceedings, and she has been criticized for her weak response to the Zika epidemic. And to make matters worse, the rainy season between January and May almost always sees a spike in mosquito-borne diseases.

Zika virus is also spreading quickly. By October cases had been reported in Colombia, El Salvador, Guatemala, Mexico, Paraguay, Panama, Suriname, and Venezuela. The region to fall victim this week is Puerto Rico.

Zika viruses has been found in the US among travelers who have been to affected regions, but there is as yet no locally transmitted case there. With no treatments or preventative medicine at hand, the US Centers for Disease Control and Prevention’s suggestion is to follow steps to prevent mosquito bites: carry insect-repellent creams, wear long sleeves and pants, sleep in air-conditioned rooms or behind windows with screens.