IN November 2019, three employees of the Wuhan Institute of Virology (WIV) reportedly became sick with a flu-like illness and sought hospital care.

Their symptoms were said to be "consistent with both Covid-19 and common seasonal illness" - a routine cold in other words - but it is unclear how ill they actually became.

The scant details so far disclosed emerged through leaked US intelligence reports obtained in May by the Wall Street Journal.

The timing fits with estimates for when the novel coronavirus behind Covid is believed to have begun circulating in the Wuhan region, the epicentre of the outbreak, and has been seized on as further evidence of the "lab leak theory".

In reality, it could be read either way.

Given that the institute is located within a couple of miles of the Huanan seafood market linked to early pneumonia clusters, it is also possible that lab workers could have come into contact with infected individuals in the community.

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The World Health Organisation has said the first confirmed case was diagnosed on December 8 2019.

However, the South China Morning Post reported in March last year that Chinese authorities had been aware of a possible 'patient zero' - a 55-year-old in Hubei province - as early as November 17, with a total of 266 cases coming under medical surveillance by the time WHO was first notified about the mysterious new disease on December 31 2019.

HeraldScotland:

It is not unusual for viruses to evolve naturally in animals and then pass to humans - a process known as zoonosis.

HIV began as a simian virus in chimpanzees and other primates in West Africa before jumping species, possibly because people ate contaminated bushmeat or hunters were infected through cuts and bites while trying capture the animals.

More than 18 months on there is neither proof that SARS-CoV2 originated naturally in bats and passed to human via pangolins (the main zoonosis theory), nor proof that it had been cultured in a lab before accidentally - and catastrophically - escaping.

The latter has been gathering steam in recent weeks, however, both politically and in some scientific circles.

At the end of May, President Joe Biden ordered US intelligence officials to "redouble" their efforts to determine the source of Covid "including whether it emerged from human contact with an infected animal or from a laboratory accident", and report back to him within 90 days.

Anthony Fauci, the US government's top infectious disease expert, now says he is "not convinced" the virus emerged naturally.

HeraldScotland: Anthony Fauci, the director of the US National Institute of Allergy and Infectious Diseases and the chief medical advisor to the presidentAnthony Fauci, the director of the US National Institute of Allergy and Infectious Diseases and the chief medical advisor to the president

This marks a significant change of tone for a hypothesis that had been largely stolen and spun by Trumpian cranks and conspiracy theorists who claimed the virus was released intentionally as a bioweapon improbably engineered to kill westerners, but not Chinese people (even though at least 4,600 Chinese citizens have died with Covid).

Ironically researchers at the Wuhan institute had been collecting coronavirus samples from bats for years precisely to prevent a repeat of the SARS 2002 pandemic which killed nearly 800 people worldwide and was eventually traced to horseshoe bats in China's Yunnan province.

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So far, scientists have determined that the genome of SARS-CoV-2 is most similar to that of RATG13, a coronavirus first found in a horseshoe bat in caves near Wuhan in 2013.

Proponents of the lab leak theory contend that WIV scientists had been manipulating or "souping up" coronavirus strains and that, since this type of research was already controversial pre-pandemic, there was all the more reason to cover up a leak as the global death toll climbed.

There has also been growing suspicion about the absence of a definitive animal source (RATG13 is close - but not close enough), although a report posted to the preprint server bioRxiv on May 27 noted the discovery of a new bat coronavirus in southern China, RmYN02, which might be more closely related to SARS-CoV-2.

HeraldScotland:

However, Nobel prize-winning US biologist David Baltimore recently caused a stir by describing a feature of Covid-19's genome, known as the furin cleavage site, as “smoking gun” evidence that the virus had been genetically modified by humans.

Professor Baltimore told the Bulletin of the Atomic Scientists in May that its unusual features "make a powerful challenge to the idea of a natural origin for Sars2", but backtracked this week telling the Los Angeles Times it is "very hard to determine" whether the sequence had occurred naturally or by molecular manipulation.

In a discussion on the issue this week on the BBC Newscast podcast, the broadcaster's science correspondent Victoria Gill said: "According to virologists that I've been speaking to over the past 18 months, there are no signatures in this particular virus that suggest natural selection could not have done this so I think that still remains the most likely theory."

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But Dr Philippa Lentzos, a senior lecturer in international security at King's College London' department of global health, told the programme the debate "has become much more nuanced".

"There is no concrete evidence for or against," she said.

"I think what we have seen over the past year is more and more circumstantial evidence for that lab leak theory and no further circumstantial evidence for the natural spillover."

HeraldScotland:

There was some dismay therefore when the WHO's own report at the end of May concluded that zoonosis from bats to humans via an intermediate host was "likely to very likely" the cause of the outbreak, whereas the lab leak theory was "extremely unlikely".

Furthermore the WHO appeared to give some credence to a favoured Chinese theory that the virus may have been imported into the country via contaminated frozen food, saying this was "considered a possible pathway".

Even a natural origin does not let humanity off the hook, however.

From HIV to SARS, our treatment of animals has put us at risk of zoonotic pandemics.

That is a lesson we can learn already.