The great influenza pandemic of 1918-19, often called the Spanish flu, caused about 50 million deaths worldwide; far more than the deaths from combat casualties in the World War One (1914-18). In fact, it may have killed between 3% and 6% of the global population.
At the time, pandemic influenza was new to the world and only people exposed to milder forms of influenza in earlier flu seasons (usually winter) had partial protection against this more virulent form of the virus. And since it happened in the pre-antibiotic era, heavily infected patients were likely to die from viral pneumonia and complicating bacterial infections.
The pandemic started in January 1918, overlapping with the war for nine months and persisting in its aftermath as people travelled back to their homes. Indeed, the war played a major role in its spread and severity.
A shrinking world
World War One marked a turning point in travel. Prior to 1914, few people traversed long distances, limiting the spread of infectious diseases, such as influenza, from one place to another, and country to country. Indeed, some rural people could live for years without exposure to many of the infections that were frequent in cities.
The war saw the mobilisation and movement of large numbers of troops and related personnel, both within and between continents; it also uprooted the lives of millions of non-combatants, especially in Europe. People from places far apart became more directly connected – and more liable than ever before to be exposed to any new form of the flu.
Those from previously isolated populations, such as Alaska or the Pacific Islands, were doubly vulnerable when first exposed to pandemic influenza. The outbreak in Western Samoa, for instance, killed 22% of the population, probably because it lacked protective immunity conferred by exposure to earlier forms of (seasonal) flu.
Army recruits in World War One were brought together from a wide range of backgrounds to live in close proximity in army camps, barracks, troop-ships and trench dugouts. When exposed to pandemic influenza, those from rural backgrounds were more likely to die than urban recruits (for the same reason as Pacific Islanders and Alaskans).
Regardless of background, mortality was lower for those who had been in the army for longer periods of time. This suggests that in the months and years after recruitment but before the arrival of the pandemic strain of the flu, soldiers became progressively immunised by exposures to seasonal flu. Or, to one or other of the bacterial infections that could cause fatal pneumonia as a complication of the flu.
New and deadly
But how do we account for the arrival of the influenza pandemic in the 1918? Where did it come from, why was it so deadly?
We now know the molecular nature of the pandemic virus, having pieced it together with nucleic acid analysis of viral fragments from pathology specimens and from bodies dug out of the Alaskan permafrost, where entire villages had been wiped out by the Spanish flu.
The pandemic virus proved to have some gene segments like those from pig and bird influenza, which explains why it was new to humans in 1918. But such a new virus, with animal features, would have needed to evolve further to easily spread within the human population.
Wartime conditions helped it along. As people living in close proximity became infected, and the number of infected people and viruses transmitted grew, the overall size of the viral population grew rapidly.
With many more viruses being made, there was greater scope for the emergence of new mutations that could grow and spread more readily in humans. Such rapidly-spreading variants would have quickly out-competed the slower growing forms of virus.
And they posed an even greater threat of the viral load overwhelming the immune system of those infected, and leading to severe illness or death.
A safer time
From August 1918, the virus was spreading around the world in several waves, infecting almost everyone. It caused illness in 20% to 50% of infected people and death in 1% to 5%.
Although the pandemic influenza virus lived on in the years after 1918-19, it tended to cause less severe disease in later years. This was partly because people whose first exposure was to a small dose of the virus would have had a mild attack, and their immune response after that would have provided protection against any subsequent attack.
Further, the practice of isolating people with severe influenza, which became standard once the pandemic was in full swing, would have limited the spread of the most aggressive viruses, and favoured the spread of variants causing milder illness.
Conditions have changed in the century since the World War One, making it unlikely the pandemic influenza disaster of 1918-19 will ever be repeated. With the increase in travel and improvements in health care, there are few enclaves where people are not at least partly protected by regular exposure to seasonal influenza or by vaccination.
Even if a nasty new flu virus were to emerge again, there will be fewer susceptible people to facilitate its spread and its evolution into an aggressive pandemic virus. What’s more, we now have medicines to help prevent its spread and to better treat the complications of severe infections.