By the time Boris Johnson was stepping back into Number 10 Downing Street on December 13 2019 as a newly-elected Prime Minister, Covid - a disease that would nearly claim his life and define his premiership - had already begun spreading in China.

Although the first cluster of mysterious pneumonia cases was not officially identified by the health authorities in Wuhan until December 31, scientists have since estimated that the earliest infections probably date back to October or November 2019.

At the time, Britain was in the grip of an election campaign that was all about "getting Brexit done"; no one could have envisioned the catastrophe to come.

Four and a half years on, as we find ourselves thrust once again into a snap general election, another quietly troubling virus story was unfolding thousands of miles away - this time in the United States.

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​On Wednesday, the Centres for Disease Control (CDC) confirmed that a second human case of the "highly pathogenic" avian influenza, H5N1, had been detected in a farm worker in Michigan.

The infection is associated with an ongoing, multi-state outbreak of the virus among dairy cows which was first detected in March, having previously wiped out thousands of seabirds before spreading into mammals.

In April, the first case of apparent cow-to-human transmission was found in Texas, also affecting a farm worker.

In both cases, symptoms were mild and more consistent with 'pink eye' (conjunctivitis), causing redness and inflammation.

The patients initially tested negative on nasal swabs and it was only after eye swabs were sent to the CDC that they returned positive results for H5N1 infections.

The CDC insists the risk to people remains low.

In a statement, it said: "CDC has been watching influenza surveillance systems closely, particularly in affected states, and there has been no sign of unusual influenza activity in people, including in syndromic surveillance.

"Based on the information available, this infection does not change CDC’s current H5N1 bird flu human health risk assessment for the US general public, which the agency considers to be low."

It has advised anyone working with livestock and poultry to wear gloves, masks, and eye protection, but concerns have been raised that many farm employees are yet to receive any personal protective equipment (PPE) - either because state authorities have not distributed it, or because farm owners have not asked for it.

The Herald: Human cases of H5N1 peaked around 2014/15, but have been low overall. Up to March this year, the UK had only ever reported five cases - all of them between 2022 and 2023, and all asymptomatic (Image: CDC)Human cases of H5N1 peaked around 2014/15, but have been low overall. Up to March this year, the UK had only ever reported five cases - all of them between 2022 and 2023, and all asymptomatic (Image: CDC) (Image: CDC)

Infections with H5N1 - a subtype of influenza A - are nothing new.

Human cases have been reported sporadically in 23 countries since 1997, but these were mostly the result of unprotected exposures to sick or dead infected poultry.

To date, there has never been evidence of any sustained human-to-human transmission of the virus.

Something which has tended to ring alarm bells, however, is its apparently high case-fatality rate (CFR).

Of the 911 known human cases over the past 20 years, more than 50% of those infected died.

For comparison, back in 2020 - before vaccinations and mutations - it was estimated that 1% of Covid infections resulted in death.

In reality, H5N1 may be far less lethal than that it appears once you factor in asymptomatic and mild cases where people have recovered without ever seeking treatment.

It is also possible that, were it ever to evolve into a form which enabled sustained human-to-human transmission, those same genomic changes might also dampen down its virulence.

The H5N1 strain - a distant relative of the devastating Spanish flu which killed some 50 million people in 1918-1919 - was first discovered in 1996 in a goose in Guangdong, a coastal province of southeast China which borders Hong Kong.

The Herald: The H5N1 bird flu first began spreading in domesticated flocks, like chickens, but has been killing large numbers of wild seabirds since 2021 and is spreading more widely in mammalsThe H5N1 bird flu first began spreading in domesticated flocks, like chickens, but has been killing large numbers of wild seabirds since 2021 and is spreading more widely in mammals (Image: Getty)

The virus could spread through an entire flock of domesticated birds, such as chickens, within days - wiping out stocks.

By 2003, as its ability to jump the species barrier became clear, virologists were warning that that the world was "teetering on the edge of a pandemic that could kill a large fraction of the human population".

It seemed like an imminent threat.

In the end, H5N1 rumbled away without much fanfare over the subsequent decades.

Eventually, while the world was distracted by a completely different pandemic - one caused not by an influenza, but a coronavirus - it made its comeback.

From October 2021, H5N1 began causing an "unprecedented number of outbreaks" and an "alarming" rate of deaths among wild birds, especially seabirds.

By May 2023, it had been detected in 80 different bird species, including golden eagles and gannets, and killed 40% of Scotland's skua population.

The virus began cropping up in parts of the world where it had never been detected before, and killing mammals which had never previously been infected including otters, foxes, dolphins, seals, and sea lions.

By October 2023, it was killing hundreds of elephant seals in Antarctica and a polar bear in Alaska is also known to have succumbed to the infection.

Dr Gregorio Torres, head of the science department at the World Organisation for Animal Health (WOAH) called it a "paradigm change in the ecology and epidemiology of avian influenza".

The Herald: Could the NHS cope with a flu pandemic?Could the NHS cope with a flu pandemic? (Image: PA)

Its shift into cattle emerged in March this year, after dairy farmers in Texas and Kansas reported that their cows had low appetites and were producing less milk.

Surveillance shows that the virus is now spreading between cows, possibly via milking equipment, and H5N1 has been found among herds in nine US states so far.

Tests on the infected Texas farmer worker - who has made a full recovery - show that the virus has mutated in a way that makes it better adapted to mammalian cells.

It would have to strike lucky again in the genetic lottery to come up with a combination perfectly-suited to human-to-human transmission, but of course the more it spreads among and between animal species in close proximity to humans the more opportunities there are for that to happen.

On the plus side, in terms of a pharmaceutical response we might start off on a surer footing: an existing antiviral, Tamiflu, appears to be effective in combatting the disease and there are already two candidate vaccines which appear well-matched to the strain.

Whether our already buckling NHS could cope is another question, and one which the next PM - whoever they are - will hope they never have to answer.