The exact cause of asthma is unknown. Asthma is most likely caused by a combination of genetic (inherited) factors and environmental triggers (such as allergens and infections). Asthma tends to run in families, so children whose parents have asthma are more likely to develop it themselves.
Nearly half of adults with asthma have an allergy-related condition, which, in most cases developed first in childhood. (In patients who first develop asthma during adulthood, the allergic response usually does not play a strong causal role.)
In people with allergies, the immune system overreacts to exposure to allergens. Allergic asthma is triggered by inhaling certain substances (allergens) such as:
An asthma attack can also be induced or aggravated by direct irritants to the lungs. Important irritants involved in asthma include cigarette smoke, indoor chemicals, and air pollution.
Respiratory viral and bacterial infections play a role in some cases of adult-onset asthma. In both children and adults with existing allergic asthma an upper respiratory tract infection often worsens an attack.
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