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INTRODUCTION

Asthma is an extremely common clinical syndrome and the most common chronic disease in children.1 Asthma affects approximately 235 million people worldwide2 and nearly 25 million Americans.3 In the United States, asthma accounts for over 10 million medical visits yearly.4 The estimated total annual cost of asthma care in the United States from 2008 to 2013 was more than 80 billion dollars.5 These costs include $50.3 billion in medical expenses, $29 billion due to asthma-related mortality, and $3 billion due to missed work and school days.5

In 2018, 22% of those afflicted with asthma were children under the age of 18 years,3 and most cases of asthma begin in preschool age.6 This propensity to become clinically apparent early in life belies the fact that asthma originates in utero as an abnormality of fetal lung development. Asthma has important consequences in childhood and may have important consequences for adult obstructive lung disease. In addition to having clear genomic and prenatal developmental components, asthma risk and prognosis are influenced greatly by exposures including respiratory viruses, indoor allergens, maternal tobacco smoke, and other physical and social aspects of the environment. The paradox of this illness is that despite important strides in understanding etiologic environmental factors and mechanisms of airway inflammation characteristic of the syndrome, its prevalence and clinical burden remain unacceptably high. Although asthma morbidity and mortality rates have been steady for several years and have declined slightly recently,7 the rates are dramatically higher than 30 years ago and continue to be significant, particularly for urban minority groups, low-income populations, and children.

The purpose of this chapter is to describe trends in asthma epidemiology and to examine potential reasons for these trends. We discuss the epidemiology of airway hyperresponsiveness, which is more prevalent than asthma, and review clinical heterogeneity in asthma and the concept of asthma endotypes. Environmental risk factors for asthma are discussed, with special attention to the emerging role of the human and environmental microbiomes. We conclude with a review of asthma natural history and the implications of the current trends, especially with regard to health disparities.

DEFINITION, PREVALENCE AND MORBIDITY OF ASTHMA

In 2007, the National Asthma Education and Prevention Program Expert Panel Report 3 (NAEPPR3)8 defined asthma as a chronic inflammatory disorder of the airways in which many cells and cellular elements play a role, including mast cells, eosinophils, neutrophils, T lymphocytes, macrophages, and epithelial cells. In susceptible individuals, this inflammation causes recurrent episodes of coughing, wheezing, breathlessness, and chest tightness. These episodes are usually associated with widespread but variable airflow obstruction that is reversible either spontaneously or with treatment.

Because asthma is a clinical syndrome, there is no gold standard for its diagnosis. As such, physicians employ nonstandardized algorithms for making the diagnosis, such as a history of wheezing or a parental ...

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