Mesothelioma is a rare, aggressive form of cancer that develops in the lining of the lungs, abdomen, or heart. People who have worked with or been exposed to asbestos have the highest risk of developing mesothelioma. After being exposed to asbestos, mesothelioma symptoms can take 20 – 50 years to appear. Mesothelioma has no known cure and has a very poor prognosis.