Genetics and environmental factors both lead to the development of asthma. That means: a person with a genetic susceptibility, when exposed to specific stimuli, is bound to develop asthma.
There are many environmental factors that can lead to asthma. For instance, allergens (substances that cause allergic reactions), such as dust mites, animal dander, cockroaches, and fungi, are common asthma triggers. Respiratory infections, especially those caused by viruses, are also notorious in causing acute attacks.
Even commonplace phenomena, such as exercise and rapid breathing, are enough to trigger an attack. Emotional stress alone can actually be enough to cause an exacerbation or asthma attack. Other factors include gastro-esopohageal reflux disease (GERD), allergic rhinitis, sinusitis, allergy to sulphites and non-steroidal anti-inflammatory drugs (NSAIDs, commonly used for pain and fever), beta-blocker use, pollution, cigarette smoke, and household chemical fumes.
Obesity is also a cause of asthma, according to a study by Camargo, et al. In the study, abnormal fat and sugar metabolism—both characteristics present in obesity—seem to be linked to asthma.
Finally, it is interesting to note that cleanliness is not a sure way to avoid asthma attacks: according to Anderson, et al. (2001), people who suffered from tuberculosis, measles, or Hepatitis A during childhood seem to be less prone to asthma; and a child who is unexposed to certain microbes and parasites has a higher risk for asthma.