This article is taken from the July 2023 issue of The Critic. To get the full magazine why not subscribe? Right now we’re offering five issues for just £10.

Germs have always befuddled humans. It’s not only their capacity to cause human suffering, but their ability to set us against one another. Take poor Ignaz Semmelweis, a doctor in a Viennese maternity ward of the 1840s.

He noted that maternal death was much higher in women attended to by doctors than those looked after by midwives. The midwives, he noted, washed their hands whereas the surgeons shuttled ungloved and unwashed between maternity ward and morgue. His pilot experiment of getting the doctors to wash their hands with chlorinated water showed a startling drop in maternal mortality.

A fat lot of good this insight did Semmelweis: his vocal advocacy for hand hygiene banished him to Budapest and eventually to an asylum where he died of septicaemia, even his wife believing he had lost his mind. For my part, I always advise fellow physician scientists to be wary of voicing their theories at home. Not until Louis Pasteur and others espoused “germ theory” and characterised bacteria from 1860 onwards was Semmelweis recognised as a pioneer.

It has taken a while, then, for the penny to drop. Jonathan Kennedy’s excellent book details how germs have been laying waste to varied civilisations since Homo sapiens became dominant some 50,000 years ago. The central thesis of Pathogenesis is that microbes such as bacteria and viruses have done more to shape human history than the scheming of politicians and generals ever could. This is a tale of how inadvertent biological warfare has shaped history, the sudden replacement of one group with another driven by the diseases that invaders and migrants bring with them, rather than military might or technological superiority.

The early chapters chronicle how a rich cast of characters including Neanderthals and Denisovans gives way to Homo sapiens migrating out of Africa. Kennedy describes a painstaking evolutionary process in which European Neanderthals and African Homo sapiens succumbed to each other’s local diseases whenever they met, then acquired immunity towards them through the “biohacking” of interbreeding. They thus gained genetic variants that made them resistant and able to migrate in large numbers.

What gave Homo sapiens the advantage? Climatic factors meant that were simply many more of them. What is more, they had evolved amongst a menagerie of diseases carried by the varied species found in tropical rainforests, rather than in the cleaner but harsher environment of an Eurasia emerging from the last glacial period. Whilst careful about how confident we can be on this point, Kennedy references the many recent documented examples of isolated groups being devastated by diseases imported from genetically heterogenous and disease-rich societies.

There then ensues a whistle-stop tour of pestilence. There is a recurrent theme of populations who have never been exposed to certain pathogens floundering when confronted with them. Repeatedly, their technological, military or economic sophistication offers no protection from the grim exponential spread with which the post-2020 world is wearily familiar.

In 430 BC, the Plague of Athens stymied the Athenians in the Peloponnesian War against Sparta. Trade routes, overpopulation and rudimentary sanitation created a perfect storm for the Antonine Plague that halted the Pax Romana in AD 165. Conversely, the European exploitation of Africa’s natural resources was held back by malaria until the discovery of quinine provided some protection against it.

Our hyper-aggressive immune systems leave us at risk of autoimmunity and allergy

The early slave trade between West Africa and the Caribbean brought yellow fever as well as malaria to those islands and to the southern American states, perversely ensuring that plantation owners would then only countenance relatively resistant African slaves as workers.

Kennedy has a fluent and engaging style. He showcases his interdisciplinary training over 300 pages that take in vast swathes of history. The message never flags that pathogens are what happens when we are making other plans, despite some of the inherent predictability that Yersinia pestis, influenza, measles and others impose upon it. The crushing inevitability when we get to the sophisticated though smallpox-naive culture of the Incas is akin to the start of an episode of Casualty: “just going to fix that leaky roof, love … ”

Is the case occasionally overstated, favouring the infectious variable over others? Probably. Inherently, the replacement of hunter-gatherers with farmers seems like a less complex system than the war between Athenians and Spartans, or the challenges faced by the Roman Empire over many centuries.

Where war, famine and policy get a look-in, they are seen through the lens of disease. However, the devastating speed with which nature’s most virulent pathogens can spread ensures that the essential point is always plausible. In this sense, recent history does some of Kennedy’s work for him.

This book is timely for many reasons. The Covid-19 pandemic naturally looms large; as Kennedy neatly chronicles, such things have happened many times before. I reflected on the similarities between the most ancient plagues and my own Covid memories of caring for patients with kidney disease in north-west London — the sense of siege, the treatment desert, the distrust in many quarters of governments and medical professionals.

The success of this book is also underpinned by the recent revolution in isolating and analysing ancient DNA from extinct hominins as well as the bacteria that infected them. Svante Pääbo was awarded the 2022 Nobel Prize in Physiology or Medicine for his work in sequencing the Neanderthal genome and discovering the existence of the Denisovans from a 40,000-year-old fragment of a finger bone in southern Siberia.

It is Pääbo and colleagues who have shown that the DNA sequences which confer the greatest risk of death from Covid-19 are inherited from Neanderthals. Kennedy might have dwelt more on the extraordinary advances in DNA isolation and sequencing that have made this possible.

An interesting corollary of this constant struggle with microbes is that we all harbour stretches of DNA that helped our ancestors survive diseases that killed millions. This is driven by a robust immune response to kill the invader, which must match it for speed and aggression. The specific “adaptive” responses that provide immunological memory need to nurture cells that can eliminate pathogens but not attack our own tissues.

There is, then, a potential pay-off for the swashbuckling aggression of successful immunity in the form of collateral damage. Our hyper-aggressive immune systems leave us at risk of autoimmunity and allergy: the attack of our own tissues or exaggerated immune response to harmless entities such as pollen. Ample evidence suggests that the incidence of both these disease entities has increased markedly in recent years. The life-saving measures of sanitation, vaccination and antibiotics may have let the devil find work for the immune system’s idle hands.

Kennedy finishes with some predictions about future pandemics and what we must do to prevent them. The stochastic chaos of the preceding 300 pages made me take these with a pinch of salt. In that sense, his book is a job well done.