Mesothelioma is a rare and aggressive form of cancer that affects the mesothelium, a protective lining that covers the internal organs of the body. It is most commonly caused by exposure to asbestos, a naturally occurring mineral that was widely used in construction and other industries for its fire-resistant and insulating properties. When asbestos fibers are inhaled or ingested, they can become embedded in the mesothelium and lead to the development of mesothelioma.