Malignant mesothelioma is a rare cancer that occurs in the tissue covering the outside surface of the lung (pleural tissue) or in the tissue covering the stomach and bowels (peritoneal tissue). Malignant mesothelioma nearly always results from exposure to airborne asbestos that is either inhaled or swallowed. The small asbestos fibers work their way to the outside surface of the lung or the outside surface of the bowel where they become trapped. Over many years, these trapped fibers can cause a reaction that may lead to malignant mesothelioma. The time required from first exposure to airborne asbestos fibers to the appearance of malignant mesothelioma can be twenty years or more. This time lag is referred to as the latency period.