Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: (CNN) The first human trial of a Zika vaccine will begin soon, Inovio Pharmaceuticals said Monday. Inovio, which is based in Plymouth Meeting, Pennsylvania, and GeneOne Life Science, which is based in Seoul, South Korea, worked together on the vaccine. They previously collaborated to create vaccines for Ebola and MERS, both of which are being tested. The Zika vaccine, with the clinical-sounding name GLS-5700, will first be tested in 40 healthy volunteers. The first tests in humans should start in the next few weeks, Inovio said in a news release. In the animal testing stage, the vaccine caused a strong antibody response, the company said.Inovio Pharmaceuticals and GeneOne Life Science will start human testing of Zika vaccineThe first Zika vaccine tests will be in 40 healthy volunteers 
 Question:Until recently, _ had been considered a rare and relatively benign virus (PDF).

Zika