ON FEBRUARY 1st, the World Health Organisation declared the spread of the Zika virus a global public health emergency. The virus, a suspected cause of birth defects in babies born to mothers who are infected during pregnancy, seems to have come from nowhere. But it has been known about for nearly 70 years. The virus was discovered in 1947, in a rhesus monkey in the Zika forest near the shore of lake Victoria in Uganda. Researchers studying yellow fever, another virus transmitted by mosquitoes, had put the monkey up on a tree, in a cage, as bait for mosquitoes. Tests of the animal’s blood turned up an unknown virus, which also turned up in mosquitoes in the same forest—a clue on how it spread. Why was Zika ignored for so long? 
Zika was found in a human for the first time in 1952, in Uganda. Nobody knows for sure when and where it began to infect humans. It may have been circulating among monkeys and other animals in the jungle for thousands of years, making the jump to humans when they got to be too close, too often to these animals (and their mosquito courtiers). For that, Zika is hardly unusual: six in ten infectious diseases in humans are spread from animals. Over the next five decades, Zika was documented in a handful of people in Africa and Asia. Some studies suggest that it may have been quietly circulating in parts of West Africa and Southeast Asia. But it was not of much interest to scientists—or of alarm to public health hawks—for a long time because it appeared to cause only mild flu-like symptoms and no massive outbreaks had been reported. Many dangerous new pathogens have jostled for their attention: since Zika was discovered, more than 300 contagious diseases have newly emerged or re-emerged in populations that had never been exposed to them, including HIV/AIDS, SARS, Ebola and antibiotic-resistant bacteria.
Zika may have been infecting many people in Africa for years, staying under the radar of patchy health systems and poor disease surveillance along with any serious health problems that it may have been causing (which is easier when they are very rare), says Alain Kohl of the University of Glasgow. Epidemiologists took notice in 2007 when Zika reached Yap, a small Pacific island where, by one estimate, it infected nearly 75% of the population–showing that it can be epidemic material. In late 2013, the virus went rampant in French Polynesia, a Pacific archipelago. There, health officials noticed an increase in neurological and auto-immune complications, some causing paralysis.
A surge in these complications, and the birth defects that Zika is now strongly suspected to be causing, were easier to spot in Brazil, where Zika arrived in 2015—for two reasons. The first is that many more people were infected with Zika, so spikes in rare complications, such as microcephaly, that may be linked to the virus could be noticed in a short period of time. The second reason is that the country has a good surveillance system, which was quickly directed to look for cases of Zika and the maladies that it may be causing. Now, the world’s leading disease experts are anxious to demystify the world’s newly-anointed pandemic threat.