Boris Johnson is not the first British prime minister to fall ill during a pandemic. In September 1918, with the First World War far from over, David Lloyd George succumbed to the Spanish Flu. Johnson is not the only world leader affected by Covid-19 and a century ago, a global range of politicians was also affected by influenza; while still suffering from the aftermath of his severe infection, Lloyd George visited the Paris peace conference in 1919, only to find that the French leader George Clemenceau and the American President Woodrow Wilson had both been stricken.
Comparisons between the two epidemics can serve as more than an opportunity for a challenging pub-quiz question. As current government measures strive to "flatten the curve" of the disease's escalation, and ease the iron grip of social isolation, there are fears of a second and third wave of infection. There is good reason to be cautious given what happened in the winter of 1918-19 when there was a casual consensus that the earlier wartime "spike" of infections was over.
Two more surges of influenza occurred, and many lives were lost. The possibility of a repeat of that kind of miscalculation worries not just virologists and government minsters but also those economists who are seeking for predictive certainties as they plan for post-pandemic recovery.
In considering the economics of global recovery it is intriguing to note a significant contrast between the two pandemics. Several historians have pointed to a relatively swift economic recovery from Spanish Flu in places as different as Sweden and several regions of the USA. For several nations the major immediate struggle was to overcome the impact of a destructive world war.
In the case of the coronavirus, lasting economic damage has already been wrought by national shutdowns. Arguably the economic harm has been caused not so much by the virus itself but rather by the attempts to contain it and if possible to end its grip.
The ghosts of a century ago haunt the present in several other ways. While debate still continues about the "where" and "when" of the Spanish Flu's origins, the extraordinary level of mobility caused by the end of the world war was most definitely a key factor in the latter two spikes of influenza which occurred in the winter of 1918-19. These two spikes brought the total who died in the pandemic to a figure which clearly exceeds the number who perished in the military conflict.
The virologist Professor John Oxford has noted the way that millions of home-bound soldiers filled liners, cargo ships and train-stations during demobilisation. On the allied side, these men arrived back to family reunion parties, village gatherings and victory marches where they continued to spread the virus. Oxford has suggested that "never before or since have so many young people travelled together in such overcrowded circumstances".
But if the end of the world war had been a brief period of exceptional mobility and thus of grave danger, then the 21st century, which is characterised by teeming mega-cities and a prodigious global travel industry, constitutes two decades of systemic mobility and latent peril.
The coronavirus has been able to spread like lightning across the world and it can multiply at will in densely populated urban environments such as New York, London and Wuhan.
The expansion of air travel means that between four and five billion passengers travelled by air in 2019. In fact, it is now possible for a plane to travel non-stop from the east coast of the USA to Singapore, taking a fatal virus halfway around the world in a matter of hours.
And in London, at least two million people use the crowded underground network during every working day - a massive petri dish in which disease can multiply.
Of course, the two epidemics are separated by a century of social, technical and medical change. Yet to catch a black-and-white glimpse of mask-clad faces and emergency hospital beds in old newspaper photos is to see there an eerie foreshadowing of our current crisis.
A similar glimpse is afforded to anyone who reads the advice given by public health officials during that far-off influenza outbreak, with its all-too-familiar emphasis on hygiene and "social distancing" in the absence of effective drugs or vaccination.
As a good example of these cautionary measures, the Irish Public Health Committee issued a memorandum in November 1918 which told the population of Ireland to "keep away from crowded assemblies" and "crowded rooms" and "only see the persons you are obliged to see (so as) to avoid infecting others". The public were warned that "expectorated matter may be full of objectionable microbes" and that when sneezing, individuals should always use their handkerchiefs.
"Strictest cleanliness" should be observed at all times. And if someone should start to "feel a pain in the head, or feverish" they should "go to bed, and send for a doctor".
These precautions did not prevent the amassing of an enormous casualty rate for such a small country. The Spanish Flu caused well over 20,000 deaths in Ireland, while Britain as a whole experienced well over 200,000.
There is plenty of evidence from studies of the Spanish Flu that certain social groups and professions were more vulnerable than others to the disease. As is well known, this influenza outbreak was particularly deadly among young and healthy adults, whereas elderly people have suffered most from the impact of the coronavirus.
In assessing who was most at risk in 1918 and 1919, the historian of the Spanish Flu in Ireland, Ida Milne, has argued that incidence of the illness was "not so much class-dependant as job-dependant". There were high levels of illness and death among groups such as small shopkeepers, postal deliverers and police. In each case, there was a high level of exposure to the public.
Exposure to those who were sick or potentially sick was of course a huge risk factor, as it is now, with doctors and nurses being especially prone to illness. However, Milne does indicate that newspaper coverage often over-emphasised royalty, senior politicians and the famous in their list of victims and that then, as now, such an emphasis leads to the erroneous assessment that a novel virus does not socially "discriminate".
While indiscriminate infection may be adduced at a crudely biological level, nonetheless there is evidence that in the Spanish Flu epidemic, wealth and poverty did matter.
The Norwegian historian Svenn-Erik Mamelund has found that the size of someone's accommodation, and therefore (in all probability) their level of personal wealth, made a difference to the ensuing mortality rate. The wealthy, better educated citizens of Norway had access to earlier and more accurate diagnosis, better nursing care and enough material resources to enable them to stay away from work, as well as better levels of nutrition and fewer pre-existing diseases in an era when ailments such as tuberculosis were rife.
Several decades of advanced healthcare have undoubtedly improved human well-being, in Western Europe at least, and access to cheap and available food is much more widespread than it was in 1919.
However, the chances of avoiding infection and preventing mental ill-health during the present lockdown are undoubtedly better for those who live on an upmarket suburban avenue than in a crowded high-rise urban apartment in the inner city.
In the former case, the benefits of plentiful, high-quality food delivery safely to the door can be supplemented by exercise on quiet roads or in the garden. In the latter case, a journey to a supermarket, which may or may not be situated nearby, might have to be undertaken via a cramped lift or multilevel stairway and then along a narrow pavement beside a main road. At the end of the day a global perspective is needed when considering who is and who isn't likely to perish from Covid-19. In much of the "south world", healthcare is limited, nutrition is poor, and accommodation crowded. The capacity for government agencies or charities to deal with the impact of a pandemic in these circumstances is greatly reduced.
Another contrast between the era of the Spanish Flu and that of Covid-19 may be seen in the speed with which news now travels. Yet there are significant resonances when coverage of the two pandemics is examined.
The censorship which was a factor in China's initial response to the coronavirus had its parallel in the newspaper coverage of the Spanish Flu in Britain and Ireland which involved an initial period of tight wartime censorship - no one at home "needed" to know how the disease was starting to spread among the troops.
However, as censorship relaxed, newspapers covered a wide range of stories. In an era when the local newspaper was in its heyday in Ireland, dramatic stories of local families or distant potentates devastated by the illness often carried depictions of how suddenly and with what terrifying symptoms the deaths had occurred.
Some newspaper accounts, such as this in The Irish Times during October 1918, focused on the tragic drama of the funerals:
"Yesterday, from early morning till well after midday, cortege after cortege reached Glasnevin Cemetery, sometimes as much as three corpse-laden hearses being seen proceeding up Sackville Street at the same time... (and) inclusive of the remains brought for burial on the previous day, which had been temporarily placed in the vaults overnight, there were close on one hundred bodies for sepulchre."
Such dark reportage was added to the wildfire of gossip and speculation and made still more worrisome by the failure of the experts to find a cure. Perhaps we can see it as the equivalent of the 24-hour coverage which has accompanied Covid-19, in which "breaking" news stories about stacked coffins and multiple funerals, sensationalist tabloid journalism and the unsubstantiated claims which bound on social media have all added to the potential for panic.
This effect exists despite the undoubted value of the modern media for staving off a sense of isolation through using Zoom, Facebook and many other platforms.
In seeking to understand the present pandemic, and to imagine what may happen after its abatement, it is tempting to refer to the 1920s and 1930s, a period that embraces the optimism of the Roaring Twenties, the miseries of the Great Depression and the rise of Fascism and Soviet Communism before descent into a re-run of global war.
Fears are growing nowadays that the sudden arrival of emergency rule by ministerial decree and the steady growth of "health surveillance" are paving the way for autocracies where human liberties are once more subsumed to the needs of a state that is, like the state in George Orwell's novel 1984, always at war.
This is a future world where the vulnerability of global food supply chains and the risk of imported human viruses may convince national leaders to pull up all draw bridges at state borders.
But the work of academics such as Israeli historian Guy Beiner has focused on one other key aspect of the Spanish Flu, which is the degree to which it was officially forgotten - or at least, not spoken of. The 1920s and 1930s did of course witness the growth of a pervasive war memorial culture in countries such as Britain.
By contrast, the thousands who died in the influenza pandemic, who were often of a similar age to the young men who died in the trenches, was virtually unacknowledged as a tragic heritage and as an emotional legacy at both the community and state levels.
The mass-death from disease in 1918 and 1919, for which there was no known antidote, replicated "pre-modern" cholera outbreaks and "plagues" and arguably the primal fear meant that it was best forgotten. As one survivor of the Spanish Flu explained to Ida Milne: "We did not talk about it for fear it might come back again."
Yet as Milne has pointed out, echoing the words of Professor John Oxford: "There must have been many more acts of heroism in the homes of the world in the great pandemic than in the First World War as husbands, wives and families strained to cope with the diseased, and as many families died together."
The relevance of this observation is striking. During the crisis through which the world is now living and dying, public attention has been drawn to the "heroism", the "sense of duty" and the "sacrifice" of healthcare workers, nurses and doctors. Indeed, all those who work in vital retail and cleaning jobs which have hitherto been considered mundane have been given respect within the new "war economy".
The struggle to overcome Covid-19 has not only become a "war" but the hospitals have become the "frontline" in which staff are woefully unprepared with the armoury of equipment needed to protect themselves.
In emphasising these military resonances, the UK Health Secretary, Matt Hancock, declared in a Sunday Times article that he was especially ready for the fight with this virus, noting his own military service and his grandfather's presence at the Somme.
Some commentators will see this kind of reference as fresh evidence of a kneejerk tendency to resort to old British wartime motifs such as the Dunkirk Spirit which emphasise wartime pluck and togetherness amid adversity, failure or carnage.
However, the way in which nurses and care workers are being elevated as wartime heroes is a positive acknowledgement that the struggle for life and health is the most significant battlefield and that in times of need, "ordinary" people can show immense courage and kindness.
This rediscovery of where heroism most frequently exists, and is most often unacknowledged, will also make less likely the silences that characterised public memory of the Spanish Flu and those who battled with it.
The advent of wall-to wall coverage of the current crisis and the enhanced ability of each one of us to record our experiences in an information age should also ensure that the personal narratives are not forgotten.