Mesothelioma is a rare form of cancer primarily caused by asbestos exposure. There is a higher incidence of mesothelioma among U.S. Navy personnel, shipyard workers, etc. due to asbestos exposure on Navy ships. In addition, family members and others closely associated to those exposed to asbestos may also be at risk (for example the wife who washed her husband's work clothes). The cancer typically has a latency period of 20 to 50 years. The mesothelium is a membrane that covers and protects most of the internal organs of the body. It is composed of two layers of cells: One layer immediately surrounds the organ; the other forms a sac around it. The mesothelium produces a lubricating fluid that is released between these layers, allowing moving organs (such as the beating heart and the expanding and contracting lungs) to glide easily against adjacent structures. The mesothelium has different names, depending on its location in the body. The peritoneum is the mesothelial tissue that covers most of the organs in the abdominal cavity. The pleura is the membrane that surrounds the lungs and lines the wall of the chest cavity. The pericardium covers and protects the heart. The mesothelial tissue surrounding the male internal reproductive organs is called the tunica vaginalis testis. The tunica serosa uteri covers the internal reproductive organs in women.