Immunization fortifies the immune system against infectious agents either through active immunization, where the body develops its own memory B and T cells after controlled exposure, or passive immunization, which introduces ready-made antibodies. While natural immunity arises from surviving an infection, the most common artificial method is vaccination, which safely prevents severe disease—and researchers are even exploring therapeutic cancer vaccines, as well as experimental vaccines targeting nicotine and obesity. Immunization is considered less risky than contracting a disease and is vital for both children and adults, as it protects against deadly illnesses and supports the development of a robust immune system. A landmark success is the elimination of polio in the U.S. since 1979, although the virus still poses a threat to unvaccinated travelers and people in affected regions. Before vaccines, people relied on dangerous inoculation methods, such as blowing powdered smallpox scabs up the nose in China (documented by Wan Quan in 1549), and vaccination has since been hailed as one of the "Ten Great Public Health Achievements in the 20th Century".