The English word ‘asthma’ comes from the Greek ‘asthma’, which means ‘panting with sound, wheezing’. Patients who suffer an asthmatic episode are short of breath, wheeze, cough and experience chest tightness. Given the still incomplete understanding of the mechanisms involved in asthma, ‘defining asthma is like defining love – we all know what it is, but who would trust anybody else's definition?’ (Gross, 1980, p. 203). Recent guidelines give the following definition of asthma: ‘… a chronic inflammatory disorder of the airways … the chronic inflammation causes an associated increase in airway hyperresponsiveness that leads to recurrent episodes of wheezing, breathlessness, chest tightness and coughing. These episodes are usually associated with widespread but variable airflow obstruction that is often reversible either spontaneously or with treatment’ (GINA, 2001, p. 2; www.ginasthma.com). The reversibility of the airflow obstruction, ‘spontaneously or with treatment’, is the hallmark of asthma, distinguishing it from the other two major chronic respiratory disorders, chronic bronchitis and emphysema (COPD – chronic obstructive pulmonary disease) which are largely irreversible (see ‘Chronic obstructive pulmonary disease’).

Asthma is a highly prevalent disorder. Estimates range between virtually 0% to some 60%, illustrating the wide variations in the genetic predisposition for developing asthma in different parts of the world population, and in the environmental factors eliciting asthmatic responses. In Inuit populations in North Canada the asthma prevalence is virtually zero; on the island of Tristan da Cunha it is about 57% (Zamel et al., 1996).