Asthma is the leading cause of chronic illness in children, affecting as many as 9 million children in the United States; a number that has dramatically increased over the past two decades. And though no one really knows why more and more children are developing the disease, there are a number of theories. Some experts suggest the increase may be due to limited access to care, especially those living in inner-city neighborhoods, and to poor asthma management and education. Others say children aren’t exposed to enough childhood illnesses to build up their immune system.