Coronavirus vaccine: What are human challenge trials?
Volunteers are set to be deliberately infected with Covid-19 after the UK Government said it will set up human challenge trials to speed up the development of a vaccine.
Human challenge trials are regarded by some as a faster way to develop vaccines - but they come with big risks.
They've been used for hundreds of years but a lot of people question how ethical is to deliberately expose people to an infectious disease.
More than 140 different organisations around the world are currently trying to discover a vaccine for Covid-19.
But could challenge trials be used to speed up the process? What is a human challenge trial?
BBC Health Correspondent Laura Foster explains how they work.
Video by Terry Saunders, Megan Fisher and Laura Foster.