Mesothelioma is a rare and aggressive form of cancer that develops in the thin layer of tissue surrounding vital organs known as mesothelial cells. Primarily caused by exposure to asbestos, Mesothelioma commonly affects the lining of the lungs (pleura). Still, it can also occur in the lining of the abdomen (peritoneum), heart (pericardium), or testicles (tunica vaginalis).