John Sudworth was the BBC's China correspondent when an unexplained pneumonia started worrying Wuhan doctors in December 2019. Since then, he's been investigating the origin of the virus that would turn into a devastating global pandemic.
From the beginning, there have been claims of certainty. Many scientists say the virus that causes Covid came from nature - probably carried from bats to other animals, and then to humans in a Wuhan market.
But an alternative possibility - that it leaked from a laboratory - has refused to go away. And other scientists claim there's uncertainty. For them, the mystery of Covid's origin remains unsolved.
So, where did Covid come from? For every one of the millions of lives lost, for every long sufferer, for the pain, the isolation, and the incalculable economic cost, the answer matters. It might also help us prevent another - maybe even worse - pandemic.
But it's a debate that's become politicised, toxic, and a bit crazy.
Presenter: John Sudworth Series Producer: Simon Maybin Editor: Richard Vadon Commissioning Editor: Dan Clarke</p>