(QUARTZ) – 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.

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 benign: some suffer from fever and red rashes, while others may be symptomless.

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