The most critical form of mesothelioma is the malignant form. Known as malignant mesothelioma, the primary causal factor is asbestos infection. According to the US National Cancer Institute, "Malignant mesothelioma is a disease in which cancer (malignant) cells are found in the sac lining the chest (the pleura), the lining of the abdominal cavity (the peritoneum) or the lining around the heart (the pericardium)."

Mesothelioma can be classified into three types