Watch: The Zika Virus Explained, In Three Minutes

How a relatively benign virus became a major public health threat

As the Zika virus continues to spread across the world, scientists are scrambling to understand the probable link between the virus and the rise in birth defects and other neurological conditions. Earlier this month, the World Health Organization (WHO) declared Zika and its associated conditions a global health emergency and health officials around the world have urged pregnant women not to travel to Zika-stricken areas; and for those in those regions, some officials are advising women not to become pregnant at all. Here’s what you need to know about how Zika spreads, why it’s worrying scientists, and how it turned into a global health threat.