Respiratory illnesses such as asthma have increased greatly in the past few decades. It is estimated that five percent of the population in the United States has asthma. This is a chronic disease, that is, it affects a person over a long period of time. It may begin before a child is 10 years old, or it may begin in adulthood. Childhood asthma usually disappears around the age of 20, but adults who get it later may have it all their lives.