The prevention of illness through exercise and nutrition was a small step from movements like hydropathy (which advocated the 'natural' healing powers of water), to the idea that fresh air, healthy food, and exercise could be beneficial. The physical fitness movement in America followed the influx of a large number of German immigrants who fled their country due to the 1848 revolution. The movement began there with Fredrich Ludwig Jahn, who unified exercise and sport with German history and tradition and saw a connection between mental and physical health. Charles Follen, a Jahn student, led the movement in America, organizing the Round Hill School at Harvard, which stressed rigorous mental and physical exercise. In the mid-west, the Germans established their first gymnastic institution called the Turnverein in Cincinnati in 1848. Later called the Turners, these groups developed nationally and organized outings of picnics, games, gymnastics, and celebrations of German culture. Catherine Beecher promoted physical fitness for women, and felt that corsets not only made such exercise impossible, but actually deformed women's bodies, and could even be passed on to future generations and degrade the race. She was also an advocate of improving nutrition, and an early opponent of gluttony, believing condiments on food stimulated the appetite towards excess. Others championed vegetarianism, or saw lack of sunshine as a cause of stomach discomfort. Regardless of their particular bent, all of the food reformers had a common philosophy: had eating habits led to social disorder. Like physical fitness proponents, they saw a connection between reshaping the body and reshaping American society to improve the individual and the country. The physical fitness movement declined in the years preceding the Civil War, then revived, as Americans became city dwellers and took sedentary jobs. Advocates promoted 'Muscular Christianity', a movement begun in England, which stressed that the best and most moral Christians were those with sound bodies. Indian clubs became a favorite exercise tool with entire books written for club exercises. Team sports became popular after 1865, reflecting America's growing urbanization. The most popular was baseball, and in 1879 the Cincinnati Red Stockings became the country's first professional team. By the 1900s, Luther Gulick transformed the Young Men's Christian Association (YMCA) into the epitome of 'Muscular Christianity'. It became the largest organization of urban gymnasiums and fitness centers in America. What does the passage mainly discuss?