Mesothelioma is caused by asbestos exposure.  Asbestos, once regarded as a miracle mineral, was popular due its lightweight but tough characteristics as well as for its heat-resistant properties. This naturally occurring mineral was used in many commercial and consumer products, from construction materials such as cement, roofing shingles and insulation, to consumer and industrial applications such as hair dryers, automobile brake pads and pipe insulation.
Most people with malignant mesothelioma worked on jobs where they breathed asbestos. Others were exposed to asbestos in a household environment, often without knowing it.

What happens inside the body?

The cancerous cells clump together in a malignant tumour.  As it grows, the tumour pushes against and into other organs and healthy tissue, causing symptoms.  Because the mesothelium is a flat thin lining, the mesothelioma tumour often takes a diffuse shape.
In its advanced stage the cancer metastasizes through the lymph system and spreads to other parts of the body.  It is still referred to as mesothelioma because it started in the mesothelium.