On 31 December 2019, Chinese authorities told the World Health OrganisationThe United Nations agency responsible for global public health. (WHO) about an outbreak of pneumoniaSwelling of the tissues in the lungs. It can cause coughing and breathing difficulties. around a seafood market in the city of Wuhan. By the end of January, the virus now known as Covid-19 had been spotted in 18 countries. In March, the WHO declared a pandemicAn outbreak of disease which occurs across a wide geographic area, as opposed to an epidemic, which is confined to a particular region.. Much of the world went into lockdown.

The pandemic was a once-in-a-century global crisis. So far, there have been 609 million reported cases and 6.52 million confirmed deaths, with many more estimated.¹ This makes it the fifth deadliest epidemic in history.

It changed the world in other ways, too. It became hard to travel. Shops and cafes closed. People began to spend all their time on computers learning or working digitally.

Now, 990 days after the first cases, the pandemic seems to be drawing to a close. Global weekly deaths are down to 11,118, the lowest level since March 2022. WHO director general Tedros Adhanom Ghebreyesus said, "We have never been in a better position to end the pandemic - we are not there yet, but the end is in sight." 

That is more than can be said for the beginning. The origins of the virus remain unknown.

The most common theory is that the virus spread naturally. Similar viruses to Covid-19 have been found in horseshoe bats. A bat could have passed the disease to humans, either directly or through a sequence of other animals. This process is called zoonosis.

But there is another idea. From the very start of the pandemic, a variety of voices² have pointed out that Wuhan is home to a virus laboratory. Could a specimen have accidentally leaked out?

In early 2021, a team of WHO investigators visited Wuhan. They said a lab leak was unlikely. But they also came back with no evidence that Covid had spread naturally. 

Tedros³ said that the lab-leak idea should "remain on the table". US President Joe Biden asked his country's intelligence services to launch their own hunt. The debate continues today.

Those against the leak theory point out that it has never happened before. Spillovers from nature happen all the time. Many past outbreaks, including HIVA virus that damages your immune system., Ebola, SARS and MERS, came from the animal kingdom. 

Supporters, on the other hand, argue there is still much to learn. As writer Thomas Fazi says: "Absence of evidence is not evidence of absence."

Yes: The simplest explanation is often the best one. For Covid-19 to have emerged naturally would require a long series of transmissions. The lab-leak theory is far clearer, and therefore more likely.

No: Humans like to think that they control the world. The idea that a world-conquering virus can develop without us is scary, as it suggests nature is stranger and more powerful than us. And it is.

Or... We simply do not know. None of the evidence for either side is enough to rule the other one out. This could change in time, with further research. But we may never find out.