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.
Asthma and allergies often coexist, and the allergic response plays a strong role in childhood asthma. About 70 - 85% of children with asthma also have allergies. Some studies suggest that children who have allergies are also at greater risk for developing asthma as adults. However, only a minority of children with allergies have asthma.
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 be induced or aggravated by direct irritants to the lungs. Studies indicate that the more indoor allergens a child is allergic to, the higher the risk for severe asthma. Important irritants include:
The role of early childhood respiratory and intestinal infections is very complex. Viral respiratory infections certainly worsen existing asthma, but the most common ones are unlikely to cause childhood asthma. In fact, early respiratory and intestinal infections may offer some protection against asthma.
Studies suggest that most respiratory infections are not important causes of asthma in children, except in certain cases. An important exception is the respiratory syncytial virus (RSV), which has been implicated in the development of asthma. RSV is the major viral cause of infant pneumonia. Studies also indicate that infants who have reduced lung function within a few days after birth are at increased risk of developing asthma by the time they are 10 years old.
Common respiratory infection viruses that cause colds (such as the rhinovirus) may be associated with the development of asthma in some people. More likely, these viruses do not directly cause asthma, but worsen asthma in children who already have it. Rhinovirus has been reported to be the most common infection associated with asthma attacks.
Research indicates that children who have a viral-induced wheezing during infancy may be at increased risk for later development of asthma. However, many children outgrow attacks of intermittent wheezing.
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