Asthma is a chronic condition that affects the airways. It causes wheezing and can make it hard to breathe. Some triggers include exposure to an allergen or irritant, viruses, exercise, emotional stress, and other factors. Asthma causes the inside walls of the airways, or the bronchial tubes, to become swollen and inflamed. During an asthma attack, the airways will swell, the muscles around them will tighten, and it becomes difficult for air to move in and out of the lungs. Around 7.9% of people in the United States had asthma in 2017. There are many types of asthma, and several factors can cause asthma or trigger an acute attack. This article looks at the types, causes, and triggers of asthma, as well as how a doctor diagnoses it.