The tourists stood still in horror, their binoculars all but forgotten.

Before them, an elephant was swaying. As it lumbered in ever smaller circles its eyes began to glaze over. Suddenly, without any warning, the majestic beast collapsed, tumbling face first into the dusty ground.

Then another fell, and another. Within a few weeks, 400 elephants lay dead across BotswanaA country of 2.3 million people in southern Africa known for its wildlife., the land littered with giant corpses. By early July, the lush grasslands of the Okavango delta had transformed into an elephant graveyard. Scientists are racing to find out why.

Africa's overall elephant population is declining. Botswana is a rare success story: elephant numbers have grown from 80,000 in the 1990s to 130,000 today. Now, they all could be at risk.

At first, conservationists thought poachers may be responsible - but the elephants' tusks, worth thousands of pounds, were not removed.

CyanideIn 2013, poachers in Zimbabwe used the poison to kill more than 300 elephants., placed in the water by farmers as revenge for damaged crops, was also ruled out when no other animals fell ill. And tests showed that anthrax is not to blame.

Now, scientists worry that the elephants could be infected with an unknown pathogenA bacteria, virus, or any other tiny organism that can cause disease. - a new deadly virus.

Last week, Botswana's government received reports from samples taken from the elephants. But they have refused to release the results. Officials say they are waiting for more answers from laboratories abroad. Until then, locals are being warned not to touch the carcassesThe dead bodies of animals..

The fear is that whatever is killing the elephants could harm humans too.

"We are currently living with a zoonotic spillover event," says conservationist Niall McCannAn explorer and a conservation director for the National Park Rescue charity.. "The worst-case scenario is that this could turn into another one. It is incredibly important to rule out the prospect of this crossing over into people."

McCann is not alone in his concerns. More than six months into the coronavirus crisis, world leaders are paying more attention than ever to the threat of emerging infections.

And scientists warn that the potential for a new deadly pandemic is only rising.

Up to 1.7 million unidentified viruses are thought to exist in mammals or water birds. Any could potentially be more lethal than Covid-19.

Just this month, a herdsman in Inner Mongolia fell ill with the bubonic plagueThe bacterial infection was once the world's most feared disease, but it can now be easily treated with antibiotics. Recent cases prompted Russian officials to warn remote countries not to hunt marmots, which spread the disease., which killed a fifth of London's population in 1665.

Today, deforestation and mining is forcing wildlife into human environments where new diseases flourish.

Professor Matthew Baylis of Liverpool University summed up the situation: "In the last 20 years, we've had six significant threats: Sars, Mers, Ebola, avian influenza, swine flu, and Covid-19. We dodged five bullets, but the sixth got us. And this is not the last pandemic we are going to face, so we need to be looking more closely at wildlife disease."

MegacitiesA city housing a population of 10 million or more. According to the UN, the smallest is Bangkok (10.2 million) and the largest is Tokyo (37.5 million). like Wuhan, where animals are forced to interact with millions of people, are the perfect places for infections to spread. The diseases may originate in wildlife, but scientists warn that humans are the real problem.

Yes, say some. The decision of Botswana's government not to share the test results is very sinister. Until scientists know the cause of the deaths, there is no way to prevent more elephants dying - and, crucially, to protect humans.

No, say others. There is currently no evidence of any disease in local people. Concerns of a new coronavirus-style pandemic are alarmist; it is pointless to speculate before the test results are released. The focus here should be the impact on elephants, not humans.