The exact cause of asthma isn't known. Researchers think some genetic and environmental factors interact to cause asthma, most often early in life. These factors include:
•An inherited tendency to develop allergies, called atopy (AT-o-pe)
•Parents who have asthma
•Certain respiratory infections during childhood
•Contact with some airborne allergens or exposure to some viral infections in infancy or in early childhood when the immune system is developing
If asthma or atopy runs in your family, exposure to irritants (for example, tobacco smoke) may make your airways more reactive to substances in the air.
Some factors may be more likely to cause asthma in some people than in others. Researchers continue to explore what causes asthma.