Covid, influenza and HIV all caused pandemics because of this one key thing

"It is not a question of  'if' another pandemic will strike but 'when'. The evidence is overwhelmingly to the effect that another pandemic – potentially one that is even more transmissible and lethal – is likely to occur in the near to medium future." An unexceptional statement of fact in the Covid Inquiry Report. But nobody knows when it will start or what will be its cause. Neither can we be certain about how it will be transmitted – by breathing, eating, touching, insects, or sex. Will globalisation be to blame? Nothing new. Columbus's sailors started a syphilis pandemic that still runs. And closer contact between humans and animals? Less now than during the Black Death.

Pathogens evolve in real time, and our ignorance of the forces that drive this process in the field is profound. But gloom must be tempered by historical successes. In the UK we reduced the number of cases of diphtheria to penny numbers after the great pandemic of 1855-63 by immunisation. But it took a long time, not preventing the surge of cases in Scotland in 1939-40 that included June Almeida's young brother, whose death drove her interest in science, leading to her discovery of human coronaviruses.

Pandemic planners have traditionally focused on influenza, a rational decision because it caused pandemics in 1889-94, 1918-20, 1957-59, 1968-70, 1977-78, and 2009-10. But, to quote the virus, "oh, what a tangled web we weave, when first we practise to deceive".

Mexican pigs, not Chinese birds

In 1976, 40 million were vaccinated against swine flu in the US because two isolations of it had been made from soldiers in a camp, which were thought to presage the return of the 1918 killer flu. But it died out of its own accord, vaccine complications were keenly sought and found, and President Gerald Ford, who had publicly backed the programme, lost to Jimmy Carter in the presidential election.

It deceived again just before 2009 when planners focused on a virus that was lethal for chickens in Hong Kong, predicting that the next pandemic virus would come from Chinese birds, building momentum before arriving in the UK, and killing at least 55,00. But it came from Mexican pigs, arrived in the UK four days after its identification there, and killed 457, leaving a misplaced, optimistic legacy for pandemic planners just before the arrival of Covid-19.

Care home tragedy

Pandemic prediction is a fool’s game. But planning for the next one is essential. Whenever a new human-infecting microbe appears, measuring ‘theta’, the proportion of cases contracted from infected individuals before they develop symptoms, or never have them at all, is crucial.

It is high for Covid-19, influenza and HIV. That is why they caused pandemics. For Covid-19, it took too long to be measured, with tragic results for care home residents. It was very low for Sars, which didn't cause a pandemic because it could be quickly and effectively controlled by isolating those with symptoms. And for planners, it goes without saying that establishing surge capacity to deal with a sudden rise in infections is essential, memories of PPE and intensive care difficulties are still fresh. Money is tight. I wish them luck.

Hugh Pennington is emeritus professor of bacteriology at the University of Aberdeen