The Mayo Clinic, representing more than 3,300 physicians, scientists and researchers, says that it isn't clear why only some people get asthma.
According to their research the cause of asthma is genetic and environmental. This means it is passed on through families and triggered by environmental exposure. Of course, there are cases that are purely hereditary and cases that are purely environmental. For example, a chronic smoker can get asthma even if they have no genetic disposition. For average people, the main concern is to eliminate the environmental causes of asthma, especially for young children who may develop the disease from exposure. There is actually 3 things you can do to greatly reduce the environmental exposure to irritants that cause asthma.
http://9jahealth.com.ng/things-can-asthma/