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About 12 million Americans have asthma, which means someone you know probably has the disease. Even though it is so common, doctors don't know what causes asthma. They do know it isn't contagious. Asthma usually strikes during childhood. Half the children who get asthma outgrow it by adolescence. The other half spend their lives using medications and avoiding things that trigger attacks. An attack happens when something irritates an asthma sufferer's respiratory system, triggering a series of events that make it difficult for the sophisticated structures within the lungs to get oxygen into the bloodstream. An asthma attack begins by striking the bronchi, the two large tubes that connect the windpipe to the lungs, and the bronchioles, the many little tubes that carry air from the bronchi deep into the lungs. In normal lungs, air from the bronchi moves into tiny air sacs called alveoli. Oxygen moves from these sacs into the bloodstream through tiny blood vessels called capillaries. At the same time, carbon dioxide is removed from the blood and exhaled from the body. An asthma attack causes the muscles surrounding the lungs' airways to tighten. The airways can also become inflamed and swollen, making breathing much more difficult. Finally, the lungs increase production of mucus that clogs the airways even more. Asthma victims often make wheezing sounds and cough as they struggle to breathe and clear out the excess mucus. For someone with asthma, breathing out, or exhaling, is as hard as breathing in. Scientists don't think asthma is inherited, but they suspect genes that make it easier for allergies and other environmental irritations to develop into asthma are passed on from parents to children. If neither parent has asthma, you have a 10 percent chance of developing it. If one parent has asthma, your odds increase to 25 percent. If they both have it, you have a 50 percent chance of developing the disease. Many things bring on asthma attacks and these triggers vary from person to person. Cold winter air, cleaning solvents, dust, spicy food, aspirin, and cigarette smoke can all be triggers. Exercise and strong emotions also can cause attacks. So can viral and bacterial infections. With so many triggers, how can people with asthma live normal, active lives? Most do by inhaling medications that dilate, or open, constricted airways and stop inflammation. They also learn what their specific triggers are and try to avoid them. A cure isn't on the horizon, but people with asthma can control the disease and turn it into an inconvenience, not a barrier to a full life.