The prevalence of inflammatory bowel disease, that is Crohn’s disease and ulcerative colitis, has increased over two to three generations in high-income countries, but in only one generation (the past 25 years) in much of the newly industrialised and “developing” world. In China, for example, these conditions have changed from being rare to common and now account for the use of as much as a quarter of gastroenterological and colorectal surgical hospital beds. 1 These diseases are not necessarily less severe initially; in developing countries inflammatory bowel disease has as severe a phenotype, with the same complications, morbidity, and mortality as in established patient populations in high-income countries. 2, 3 Speaking to physicians in many parts of the developing world, it is clear that inflammatory bowel disease has become a major health concern and an area of increasing health-care need. In their systematic review in The Lancet, Siew Ng and colleagues4 have accurately captured the size of this problem and its course. By amalgamating and analysing credible population-based studies of the incidence and prevalence of inflammatory bowel disease from around the world, 4, 5 the authors show that inflammatory bowel disease now affects between one in 200 and one in 300 people in high-income country populations. The prevalence in low-income and middleincome countries is still much lower than this, but the incidence is increasing rapidly. Since 1990, the incidence has been rising in Africa, Asia, and South America, including, for example, Brazil (11% annual increase for Crohn’s disease and 15% for ulcerative colitis) and Taiwan (4% and 5% annual increases, respectively). The rapidly increasing incidence and the huge population at risk means that the health burden related to inflammatory bowel disease in these countries is certain to increase further. If current epidemiological trends continue, the number of people affected by inflammatory bowel disease worldwide is projected to be in the tens of millions within a few decades. 6, 7 The number affected in low-income and middle-income countries might equal or even exceed that in high-income countries. 6, 7 Valuable epidemiological data such as those presented in this Article4 affect medical practice, health-care planning, and the discovery of disease causation. In much of the developing world, where infectious diseases remain common, the differentiation of inflammatory bowel disease from parasitic and bacterial infections, including enteric tuberculosis, remains a challenge. 3 Increasing recognition of its prevalence provides greater confidence in the diagnosis of inflammatory bowel disease, with better clinical outcomes. Knowing the incidence and prevalence of inflammatory bowel disease is a crucial first step in global health-care planning. All areas of health care are affected by the rapid emergence of such diseases, including the provision of trained medical and allied professional manpower, hospital beds, and expensive medications that constitute a large part of the cost of care. 7 Developing countries will need to acknowledge the importance of these data and incorporate them into their health-care planning.

Last but not least, epidemiologists are the keepers of many of the most important clues into disease causation. They are uniquely positioned to point scientists and clinical researchers in the right direction for their investigative search. Although the peak incidence of inflammatory bowel disease onset is in the first three decades of life, the die is cast for the development of inflammatory bowel disease in early childhood. Exposure to environmental factors in childhood seems to be …