China reported its first cases of the new coronavirus in December. But was Covid-19 already silently circulating? To find out scientists are looking for "patient zeros" by tracking the evolution of the virus itself.
This genetic detective work is tracing the family tree of the coronavirus that has killed tens of thousands in its relentless spread across the planet.
It could also help find out if the virus was spreading in other countries before the first infections were officially recorded.
In France, a cluster of cases was discovered in late January. But a new study published in the International Journal of Antimicrobial Agents has suggested the virus was already in the country a month earlier.