A few years ago health workers researching eye disease in rural Kenya noticed something peculiar. When they tried to measure one woman’s blood pressure, the machine would not give a reading. A nurse thought there must be a technical fault. They tried several times and then took the woman’s blood pressure manually. They found it was so high that the automated equipment could not record it.

Chances are that this old lady is no longer alive. However, her medical situation puts her at the forefront of a largely unremarked change in the history of human health. The infectious diseases that have traditionally killed the world’s poor are starting to recede. Instead, people in Africa, India and China are beginning to die of the same things that kill westerners: chiefly, obesity- and smoking-related diseases. This change is known as the “epidemiological transition”. How it plays out over the next few years will determine whether people in poor countries can for the first time expect to live into old age or whether they will simply start dying of different things.

The epidemiological transition was first described in 1971 by Abdel Omran, an ­Egyptian-born professor at the University of North Carolina. Writing in the Milbank Memorial Fund Quarterly, he drew a map of disease through human history in which he charted this gradual replacement of infectious with chronic diseases as the main causes of death. In Omran’s first stage of history, “the age of pestilence and famine”, people mostly die of infectious diseases. Life expectancy is often below 30 years. Next is the stage of “receding pandemics”. Infectious diseases come under control, although they remain the ­dominant cause of death. The average person’s life-span reaches the fifties. The third stage – which Omran thought was the last – was that of “degenerative and man-made diseases”. This is when infectious diseases diminish further and, as people live longer, they die of non-infectious diseases such as heart disease, cancer, chronic respiratory illnesses or diabetes. All these chronic diseases can originate when the victim is young, and take a long time to develop. This slow evolution affords medicine greater scope to prevent or intervene.

The epidemiological transition from infectious to chronic diseases began in western countries in the early 1900s. At that stage medicine had little to do with it. Rather, people’s health improved as they ate better and acquired flush toilets and baths, lessening the incidence of infectious ­diseases such as cholera and dysentery. By the 1950s, Europe and North America had reached Abdel Omran’s final stage of “degenerative and man-made diseases”.

We now know that Omran failed to foresee a fourth stage of the transition: the decline of chronic diseases. The west – and particularly its richest inhabitants – has now reached this stage. Thanks to the “cardiovascular revolution” – the medical advances in treatment – the past 30 years have seen death rates from heart disease fall by 70 per cent in the US, the UK, Australia, Canada and Japan. That translates as 14 million American and eight million British lives saved between 1970 and 2000.

The rest of the world is now proceeding through the phases of epidemiological transition. The researchers who were surveying blindness in Kenya had noticed this even before they met the woman with the spectacular blood pressure: they had expected to find that rural Kenyans were going blind from cataracts and infectious diseases. Instead, a surprising number of people had the same diseases of the back of the eye that are common in rich countries.

Evidence shows that infectious diseases that have killed poor people for centuries are being stamped out. The template is smallpox. In the 18th century, the disease killed every 10th child born in Sweden or France. In 1796, Edward Jenner discovered a vaccine, but by the early 1950s there were still about 50 million cases worldwide each year. Millions of the victims died, and many of the others were left badly disfigured. It seemed unthinkable that the disease could ever be eradicated.

Yet it was. The World Health Organisation was among a number of health agencies to launch a global drive to vaccinate children. By 1967, the number of cases had fallen to 15 million a year. And in 1978, in a hospital in Solihull outside Birmingham, Janet Parker became the last casualty of the disease. Parker, a medical photographer at Birmingham University, often worked above a lab where the smallpox virus was grown for research purposes. Some of the virus had apparently escaped into the lab’s air ducts.

Smallpox is the only human infectious disease that medicine has managed to eradicate thus far. However, some other diseases may slowly be following it towards the dustbin of history. For instance, polio victims with withered limbs have long been a common sight in poor countries. In 1988, there were still an estimated 350,000 cases of polio around the world. Then a global vaccination effort, led by the WHO, Unicef and the Rotary Foundation, was launched to eradicate it. By 2006 the figure had fallen by 99 per cent to 1,997 cases.

It’s a similar story for measles. A health researcher in Kintampo, Ghana, bridles at the suggestion that anyone might view the disease as a mere nuisance. “I had seven brothers who died of measles in one week,” he says. (His father had more than 50 children.) In 2006, measles still killed 242,000 people around the world, the overwhelming majority of them young children. That is 27 deaths an hour. However, this was 68 per cent below the estimated tally for 2000. In African villages, such a decline makes an untold difference. Malaria – estimated to kill a million people a year, and to affect 300 million more – appears to be declining too. In the coastal town of Fajara, in Gambia, the number of people testing positive for malaria fell between 2003 and 2007 by 73 per cent. Seven out of 115 deaths recorded in the town in 2003 were attributed to the disease; in 2007 there were none. There were similar patterns in other Gambian population centres – and these trends were mirrored on the other side of the continent, in east Africa. Deaths plunged in Kenya, Eritrea and Rwanda. There have even been reports of malaria researchers in some African countries struggling to find sufficient severe cases to research.

Today, economists continue to debate whether aid does any good. Much as smallpox once did, malaria seems to offer incontrovertible proof that it can. Much of the credit for curtailing the disease appears to belong to the “Roll Back Malaria Partnership”, launched 10 years ago by the WHO, Unicef, the United Nations Development Programme and the World Bank, and funded partly by the Bill and Melinda Gates Foundation. The partnership has helped distribute new drugs and mosquito nets. (Climate change may have been a factor, too, but was probably not decisive.)

There is even good news about the HIV/Aids pandemic. In 2007, an estimated 33 million people had HIV/Aids, two-thirds of them in sub-Saharan Africa. But a third of those cases are now being treated, far more than before. The number of people with HIV worldwide has stabilised since 2000. Even in sub-Saharan Africa, most national epidemics have either stabilised or begun to fall. Again, aid seems to have helped. Funding for HIV programmes in low- and middle-income countries rose six-fold between 2001 and 2007, partly thanks to the contribution made by the “President’s Emergency Plan for Aids Relief” initiative of George W. Bush, launched in 2003.

That was the good news. The bad news is that poor people are now dying of the chronic “western” diseases they never used to get. In 2005, about 58 million people died around the world. Although it is notoriously hard to establish causes, especially in poor countries, it is estimated that 28 million of those died in low- and middle-income countries of heart disease, strokes, cancer and other chronic diseases. By 2020, it’s projected that non-infectious diseases will be responsible for seven out of every 10 deaths in these countries. In Mexico, for example, three-quarters of all deaths are already in this category.

It was inevitable, according to Abdel Omran’s theory, that people in poorer countries would start dying of chronic diseases. What is distressing is how young they are when they do so. Half of all deaths from chronic diseases in low-income countries kill people under the age of 70, compared with only a quarter of such deaths in rich countries. This means that ­people are often hit at the peak of their economic productivity.

What seems to be driving this early mortality is the speed with which those in developing countries have adopted the least healthy habits of the west. This is particularly true of urban and wealthier classes. In China, where business relationships are often cemented with gifts of packets of cigarettes, with each brand having its own connotation, the great scourge is tobacco. The majority of Chinese men smoke. The same unhealthy behaviours cause diseases in the same way across the globe. Today, only half of new cancer cases occur in developing countries, but as their citizens start smoking more and westerners smoke less, the developing world’s share of new cancer victims will inevitably exceed those of the west.

Elsewhere the problem is often obesity. On a shopping street in Kampala or Johannesburg or Hyderabad today, you will find people as fat as those you’d see in a midwestern American mall. That wasn’t the case 10 years ago. But now, many formerly poor people can afford to gorge on calories, often in new fast-food restaurants. Many now drive instead of walk and spend hours watching television or sitting behind computer screens.

For many of these people, obesity may not yet carry the stigma that it does in rich countries. Indeed, they may view it favourably. Devina, a clinical officer in Nakuru, Kenya, announced to one of us that she had decided to become very fat. Why? “Because I think it looks nice.” In 2000, for the first time, there were more overweight than underweight people in the world.

The obesity epidemic blurs the distinction between infectious and chronic diseases: the way obesity spreads might actually mirror the transmission of infectious disease such as cholera. Whereas cholera is passed on through bacteria, obesity “travels” through social networks. A US study repeatedly weighed a network of 12,067 people over a period of 32 years. It concluded that a person’s chance of becoming obese rose as those close to him became obese. For instance, if a person’s sibling grew obese, the person’s own risk of obesity increased by 40 per cent. Perhaps the networks were changing people’s norms about what weight was acceptable; or perhaps people were simply eating and drinking together.

The obesity epidemic is now spreading rapidly in many poor countries. One consequence is the global increase in hypertension – high blood pressure – which can cause heart disease or strokes. In African ­cities, the prevalence of hypertension in adults now approaches the levels of high-income countries. The obesity epidemic in the developing world is an only slightly tamer rerun of the western epidemics that reduced Mexico’s population by nearly 90 per cent after Spanish conquistador Hernán Cortés invaded in the 16th century.

In India today, the big problem is diabetes. Not long ago, public health officials considered this a disease of relatively minor importance. That has changed, mainly as people have become fatter. A diabetes epidemic typically follows an obesity epidemic with a lag of about 10 years. Already in 2000, there were about 171 million diabetics on the planet, or four to five times as many as those living with HIV. India now has perhaps 32 million diabetics, most of whom do not know that they have the condition; China has 40 million. An extreme case is the Pacific island of Nauru, where half a century ago diabetes was almost unknown. Now 40 per cent of adults have it.

Diabetes is very much a disease of the cities. It is rife in India’s boomtown of Hyderabad. Make the slow, laborious drive out of the clogged-up city into the neighbouring villages, and the much thinner rural population is less likely to be diabetic. The problem is only partly the traditional Hyderabad biryani dish, made with meat, rice and lots of oil or ghee (clarified butter). Rich Indians now get a far larger proportion of their energy from fat than poor Indians do. A national survey found that by 2000, 12 per cent of urban Indians over the age of 20 already had diabetes.

It’s not just that people in poor countries are adopting unhealthy habits. Once ill, they are much less likely than those in rich countries to see a doctor and receive treatment. A survey in Egypt published in 2000, for instance, showed that one in three people with very severe hypertension didn’t even know they had the condition. Even if they knew, they struggled to find doctors. In Uganda, there is only one doctor of any kind for every 20,000 people, compared with one for 500 in the UK.

A theory is emerging that people in poor countries are more vulnerable to chronic diseases. The reason might be that they often have low birth weights. The “thrifty phenotype” hypothesis argues that underweight babies are programmed to expect food scarcity and so store fat very readily. When they find food in ample supply later in life, this might increase their risk of obesity. Those with low birth weight certainly appear to be particularly vulnerable to chronic diseases such as diabetes and heart disease. Also, people in poor countries might suffer worse once they acquire a chronic disease. For instance, hypertension appears to affect Africans more severely than sufferers on other continents. South Asians seem to develop heart disease four to five years earlier than their white counterparts, and their diabetes seems to be more aggressive. The reasons for this are still unclear.

When Omran wrote his article on the epidemiological transition in 1971, life expectancy had never fallen for long, except during periods of famine and war. That has since changed. We now have two documented episodes of declines in life expectancy. And one of them may be a terrifying warning for the developing world. The first decline occurred in Africa after Aids. By 2002, 22 million people had died of the disease. Life expectancy in southern Africa fell by as much as 10 years: in Botswana it dropped from 59 in 1995 to 49 in 2005.

But the second decline in life expectancy is also alarming for today’s Chinese, Indians and urban Africans. This was the decline that hit eastern Europe after the Soviet Union’s collapse. Health services and established social structures fell apart, and stress and depression increased. One result was that alcoholism soared. By 1992, some of the new kiosks along Moscow’s boulevards sold a liquid advertised as “100 per cent alcohol”. Other Russians drank eau de toilette or medicinal alcohol. In 1990, the average Russian man’s life expectancy had been 64 years. By 2005, it was just 59. A study published in The Lancet last year, conducted in Izhevsk, a typical Russian city, between 2003 and 2005, showed that 43 per cent of all deaths in men of working age were due to hazardous drinking. If these figures were extrapolated for Russia as a whole, it would translate into 170,000 excess deaths a year in Russia for men aged 25 to 54.

Chronic diseases are unlikely to afflict Chinese, Indians and Africans as horribly as they have Russian men. Life expectancy in these regions is unlikely to fall. However, Russia does point to a possible future for other poor countries: one in which the stamping out of infectious disease fails to improve life-spans. Life expectancy is currently forecast to increase in all regions of the world through to 2030, with Africa and south Asia gaining most. But it’s possible to imagine a much gloomier scenario along Russian lines: that as the world’s poor start smoking and over-eating and stop exercising but don’t have access to doctors, they will not realise all the potential gains in life expectancy.

For the first time in history, poor countries are now facing a dual burden of infectious and chronic diseases. But there is hope. The WHO has set a global goal of reducing the death rates for chronic diseases by an additional 2 per cent a year between 2005 and 2015. That would save 36 million lives. Simple changes could help achieve that target. Cutting down on smoking and salt could save nearly 14 million lives in 23 countries over this period, at a cost of less than 27 pence per person per year. Another 75 pence per person per year would buy enough aspirin and other drugs to lower blood pressure and cholesterol and avert 18 million deaths. It can be done.

Hannah Kuper is a senior lecturer at the London School of Hygiene and Tropical Medicine. Simon Kuper is an FT writer based in Paris

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