In 2016, panic spread through the Americas as a once obscure virus—Zika—reached epidemic proportions. To the surprise of the medical community and the public alike, the little known virus was discovered to cause severe birth defects in children born to infected mothers. But where did this virus come from? And how did it suddenly become a problem of such devastating proportions? A year after the devastating disease exploded in the Americas, NOVA takes a closer look at how this virus grew to become such a monster. We join a small band of researchers who knew about Zika before it made headlines as they trace the disease back to its origins… back to the tropical forests of Uganda, where it was discovered during a golden age of virus discovery.