Researchers in England are deliberately exposing volunteers to the coronavirus that causes COVID-19. The goal is to speed up the development of new vaccines and treatments. But exposing people to a potentially fatal disease with no particularly effective therapy strikes some as unnecessary, if not unethical. Human challenge experiments differ from other studies of COVID-19 in a very important respect. "The main difference is the control," says Christopher Chiu, an infectious disease researcher at Imperial College London and lead scientist for the challenge study. He says with a challenge study, you know exactly when a person was exposed to the virus, and exactly how much virus they were exposed to.