Asthma is a long-term problem in the tubes that carry air into your lungs that can make it hard for you to breathe. These airways get so narrow that air can't move freely. It can cause serious wheezing and breathlessness, known as asthma attacks. There's no cure, but most people can control their symptoms. The number of people with asthma has been going up steadily in the U.S. for decades. Some researchers suggest it's because of better hygiene: The theory is that fewer childhood infections lead to under-developed immune systems, which means a greater risk of asthma. It might also be because we use more household cleaning sprays, we don't use as much aspirin, or we have lower levels of vitamin D.