In a clip released this morning on CNN, Dr. Robert Redfield, the former director of the US Centers for Disease Control and Prevention, told CNN's Dr. Sanjay Gupta he believes the origin of the Covid-19 pandemic is a lab in China — a controversial theory without evidence.

A World Health Organization team exploring origins of the coronavirus in Wuhan, China, is expected to release a report of its findings soon, but has already said a lab-related incident is “extremely unlikely.”

The most likely causes of transmission of the Covid-19 virus to humans, the WHO team said, are introduction through an intermediary host species or transmission through the trade of frozen products. The team has also investigated direct transmission from an animal reservoir to human.

In early February, World Health Organization expert Peter Ben Embarek said that the team investigating the origins of the coronavirus in Wuhan have identified two scenarios that most likely caused the transmission of Covid-19 to the human population.

"Our initial findings suggest that the introduction through an intermediary host species is the most likely pathway and one way that will require more studies and more specific targeted research," Embarek said during a news conference.

He added that the possibility of transmission through the trade of frozen products was also likely.

Embarek also noted two other hypotheses the team had probed while investigating the origin of the virus.

One hypothesis was a "direct zoonotic spillover," meaning, direct transmission from an animal reservoir to a human. 

"The hypothesis of a direct spillover from an original animal source into the human population is also a possible pathway and is also generating recommendation for future studies," he said.

The fourth hypothesis was the possibility of a laboratory-related incident, but that this was the least likely of the four to be the cause of the virus' introduction to humans.

"Findings suggest that the laboratory hypothesis is extremely unlikely to explain the introduction of the virus to the human population," Embarek said.

Meanwhile, Chinese officials and state media have promoted an unsubstantiated, so-called “multiple-origin” theory, suggesting the pandemic may have started in various locations around the world, even a US military lab.