On 18 April 1947, a rhesus monkey that researchers identified as 766 ran a fever of 39.7°C, about 2°C higher than normal. The monkey was part of a study hunting for yellow fever virus and was living in a cage on a platform built into the tree canopy in the 1.5-kilometer-long Zika Forest, which runs adjacent to an arm of Lake Victoria in Uganda. Three days later, the investigators took a blood sample from Rhesus 766 and injected it into the brains of Swiss albino mice. The mice “showed signs of sickness” after 10 days, and the researchers harvested their brains, from which they isolated a “filterable transmissible agent.”
Come January of the following year, the same researchers trapped mosquitoes from these canopy platforms and took their bounty back to the lab, hoping to isolate yellow fever virus. Others had shown that one of these species they caught, Aedes africanus, shuttled the yellow fever virus, so the investigators put 86 of the insects in a refrigerator to “render them inactive” and then ground them up in a blood-saline solution, which they again injected into the brains of mice. The animals “appeared inactive” after 7 days, and tests showed they harbored the same transmissible agent that had sickened Rhesus 766.
The researchers called their “hitherto unrecorded virus” Zika.