Aerobic exercise is exercise that takes an extended amount of time—usually ten minutes or more—but is usually performed at only moderate intensity. Running, swimming, bicycling, and cross-country skiing are classic aerobic exercises. In 1968 a best-selling book called Aerobics introduced a system of exercise for increasing the body's ability to take in and use oxygen, and today aerobics classes, often mimicking such outdoor exercise as running and bicycling, take place every afternoon in thousands of gyms and YMCAs across the country. Aerobic exercise particularly strengthens the heart and lungs, but usually has many other good effects as well. Aerobic bacteria, which need oxygen to live, are essential for breaking down living matter so that it returns to the soil. They include the famous intestinal E. coli, as well as the staph and strep bacteria that can make a visit to the hospital risky.