Asthma is a common, (and currently incurable) long term inflammatory disease of the airways of the lungs, characterized by variable and recurring symptoms such as reversible airflow obstruction, and bronchospasm. Episodes of asthma frequently feature wheezing, coughing, chest tightness, and shortness of breath.

It's thought to be caused by a combination of genetic and environmental factors - but the exact biological mechanisms are unknown. See : Wikipedia

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

Notes :

• 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. (see :Auto-immune disease + allergy increaseplugin-autotooltip__plain plugin-autotooltip_bigAuto-immune disease + allergy increase
  
  "Since the 1950s, rates of multiple sclerosis , Crohn's disease, type 1 diabetes, and asthmahave soared by 300% or more. Similar graphs depict concurrent spikes in hay fever and food allergies." [our hyperlinks]
  
  Source : Proc Natl Acad Sci U S A. 114(7): 1433–1436. "
  
  This huge increase in auto-immune diseases and)
• "Anxiety and depression occur more commonly in people with asthma than expected, and are associated with poor asthma outcomes. The direction of the relationship and the mechanisms underlying it are uncertain." Source :Primary Care Respiratory Journal 20(3);

Also see : Exercise-Induced Asthma (EIA)plugin-autotooltip__plain plugin-autotooltip_bigExercise-Induced Asthma (EIA)

Exercise Induced Asthma, or EIA (also known as Exercise-Induced Bronchoconstriction, EIB) is a form of Asthma (constriction of the airways) which is triggered by strong exercise. It was first reported in 1989.

It's prevalent amongst professional athletes at a far higher level than the general population. For example, some studies have put the prevalence as high as 45% for cyclists and 55% for skiers and skaters,