Why have 14,000 people volunteered to be infected with coronavirus?

Imagine being told to inhale a nasal spray full of coronavirus. More than 14,000 people in the U.S. and elsewhere are putting their names forward to do so. They are volunteering for what's called a "human challenge trial," an ethically controversial way to test vaccines that would deliberately infect people with a virus that has killed over 270,000 people worldwide and has no cure.