In the twenty-first century, life is different thanks to the four public health innovations of the nineteenth and twentieth centuries, and so disease is different too. But our twenty-first-century illnesses are not simply another layer of ill-health, hidden beneath infectious disease, but an alternative set of conditions, created by the way we live now. At this point you might be wondering how these illnesses can possibly have something in common, such a disparate group do they seem. From the sneezing and itching of allergies, to the self-destruction of autoimmunity, the metabolic misery of obesity, the humiliation of digestive disorders and the stigma of mental health conditions, it’s as if our bodies, in the absence of infectious diseases, have turned on themselves.
We could accept our new fate and be grateful that we will, at least, live long lives free from the tyranny of the pathogen. Or we could ask what has changed. Could there be a link between conditions that seem unrelated, like obesity and allergies, irritable bowel syndrome and autism? Does the shift from infectious diseases to this new set of illnesses indicate that our bodies need infections to stay balanced? Or is the correlation between declining infectious disease and rising chronic illness merely hinting at a deeper cause?
We are left with one big question: Why are these twenty-first-century illnesses happening?
At the moment, it’s fashionable to look to genetics for the source of disease. The Human Genome Project has unearthed a whole heap of genes that, when mutated, can result in illness. Some mutations guarantee disease: a change to the code of the HTT gene on chromosome 4, for example, will always result in Huntington’s disease. Others mutations simply increase the likelihood: misspellings in the genes BRCA1 and BRCA2 raise a woman’s risk of breast cancer to up to an eight in ten chance in her lifetime, for instance.
Although this is the era of the genome, we cannot blame our DNA alone for the rise in our modern diseases. While one person might carry a version of a gene that makes them, say, more likely to become obese, that gene variant could not become dramatically more common in the population as a whole inside a single century. Human evolution just does not progress that rapidly. Not only that, but gene variants only grow more common through natural selection if they are beneficial, or their detrimental effects are suppressed. Asthma, diabetes, obesity and autism bring few advantages to their victims.
With genetics excluded as the cause of the rise, our next question must be: Has something changed in our environment? Just as a person’s height is a result of not only their genes, but their environment – nutrition, exercise, lifestyle and so on – so is their disease risk. And this is where it gets complicated, as so very many aspects of our lives have changed in the last century, and pinpointing which are causes and which are mere correlations requires the patient process of scientific evaluation. For obesity and its related illnesses, changes in the way we eat are clear to see, but how this affects other twenty-first-century illnesses is less obvious.
The diseases in question offer up few clues as to their joint origin. Could the same changes in our environments that lead to obesity also generate allergies? Can there really be a common cause of mental health conditions like autism and obsessive–compulsive disorder, and gut disorders like irritable bowel syndrome?
Despite the disparities, two themes emerge. The first, clearly binding allergies and autoimmune diseases, is the immune system. We are looking for a culprit which has interfered with the immune system’s ability to determine our bodily threat level, making overreactions all too common. The second theme, often hidden behind more socially acceptable symptoms, is gut dysfunction. For some modern illnesses, the link is clear: IBS and inflammatory bowel disease have bowel disturbances at the core of their presentation. For others, although it is less overt, the connection is still there. Autistic patients struggle with chronic diarrhoea; depression and IBS go hand in hand; obesity has its origin in what passes through the gut.
These two themes, the gut and the immune system, might also seem unrelated, but a closer look at the anatomy of the gut provides a further clue. Asked about their immune system, most people might think of white blood cells and lymph glands. But that’s not where most of the action is. In fact, the human gut has more immune cells than the rest of the body put together. Around 60 per cent of the immune system’s tissue is located around the intestines, particularly along the final section of the small intestine and into the caecum and the appendix. It’s easy to think of the skin as the barrier between us and the outside world, but for every square centimetre of skin, you have two square metres of gut. Though it’s on the ‘inside’, the gut has just a single layer of cells between what’s essentially the outside world, and the blood. Immune surveillance along the intestines, therefore, is critical – every molecule and cell that passes by must be assessed and quarantined if necessary.
Although the threat of infectious disease has all but gone, our immune systems are still under fire. But why? Let’s turn to the technique pioneered by Dr John Snow during Soho’s cholera outbreak of 1854: epidemiology. Since Snow first applied logic and evidence to unravelling the mystery of the source of cholera, epidemiology has become a mainstay of medical sleuthing. It couldn’t be simpler: we ask three questions: (1) Where are these diseases occurring? (2) Who are they affecting? and (3) When did they become a problem? The answers provide us with clues that can help us to answer the overall question: Why are twenty-first-century illnesses happening?
The map of cholera cases that John Snow produced in answer to Where? gave away cholera’s likely epicentre – the Broad Street pump. Without much detective work, it’s clear to see that obesity, autism, allergies and autoimmunity all began in the Western world. Stig Bengmark, professor of surgery at University College London, puts the epicentre of obesity and its related diseases in the southern states of the US. ‘States like Alabama, Louisiana and Mississippi have the highest incidence of obesity and chronic diseases in the US and the world,’ he says. ‘These diseases spread, with a pattern similar to a tsunami, across the world; to the west to New Zealand and Australia, to the north to Canada, to the east to Western Europe and the Arab world and to the south, particularly Brazil.’
Bengmark’s observation extends to the other twenty-first-century illnesses – allergies, autoimmune diseases, mental health conditions and so on – all of which have their origin in the West. Of course geography alone does not explain the rise; it merely gives clues as to other correlates, and with luck, the cause. The clearest correlate of this particular topography of illness is wealth. A great accumulation of evidence points to the correlation between chronic diseases and affluence, from grand-scale comparisons of the gross national product of entire countries, to contrasts between socio-economic groups living in the same local area.
In 1990, the population of Germany provided an elegant natural experiment into the impact of prosperity on allergies. After four decades apart, East and West Germany were reunifying following the fall of the Berlin Wall the previous year. These two states had much in common; they shared a location, a climate, and populations composed of the same racial groups. But whilst those living in West Germany had prospered, eventually catching up and keeping pace with the economic developments of the Western world, East Germans had existed in a state of suspended animation since the Second World War and were significantly poorer than their West German neighbours. This difference in wealth was somehow related to a difference in health. A study by doctors at Munich University’s Children’s Hospital found that the richer West German children were twice as likely to have allergies, and three times as likely to suffer from hay fever.
This is a pattern that repeats itself for many allergic and autoimmune conditions. American children living in poverty are historically less likely to suffer from food allergies and asthma than their wealthier counterparts. The children of ‘privileged’ families in Germany, as judged by their parents’ educations and professions, are significantly more likely to suffer from eczema than those from less privileged backgrounds. Children from impoverished homes in Northern Ireland are not as prone to developing type 1 diabetes. In Canada, inflammatory bowel disease more often accompanies a high salary than a low one. The studies go on and on, and the trends are far from local. Even a country’s gross national product can be used to predict the extent of twenty-first-century illnesses within its population.
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