Malignant mesothelioma is a disease in which malignant (cancer) cells are found in the peritoneum (the thin layer of tissue that lines the abdomen and covers most of the organs in the abdomen) or the pleura (the thin layer of tissue that lines the chest cavity and covers the lungs).

Risk factor is anything that increases your possibility of getting a disease. In case of cancer, having a risk factor does not mean that you will get a cancer; while on the other side, not having risk factors also does not mean that you will not get cancer.