Asthma is a common, (and currently incurable) long term inflammatory disease of the airways of the lungs characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include episodes of wheezing, coughing, chest tightness, and shortness of breath.
Asthma is thought to be caused by a combination of genetic and environmental factors, but the exact biological mechanisms are unknown.
“The prevalence of asthma increased steadily over the latter part of the last century, first in the developed and then in the developing world […] Current estimates suggest that asthma affects 300 million people worldwide, with a predicted additional 100 million people affected by 2025. The socio-economic impact is enormous, as poor control leads to days lost from school or work, unscheduled health-care visits and hospital admissions. Although the development and course of the disease, and the response to treatment, are influenced by genetic determinants, the rapid rise in prevalence implies that environmental factors are critically important in the development and expression of the disease. To date, studies have explored the potential role of indoor and outdoor allergens, microbial exposure, diet, vitamins, breastfeeding, tobacco smoke, air pollution and obesity but no clear consensus has emerged.”
Source :Davidsons Principles And Practice Of Medicine 22ed 2014
Note: The proportion of asthma sufferers tends to be far higher in 'developed' ( i.e. 'Westernised' ) societies. There is currently no agreed theory as to why this is happening.