There are many additional causes of mesothelioma cancer beyond just asbestos exposure, although asbestos exposure is still obviously the primary cause.  There were studies done previously on animals that showed the Simian Virus causes mesothelioma cancer.  However, no definite proof of this virus causing mesothelioma has been shown, and the only known source of this virus is it was previously supposedly found in polio vaccines decades ago.

There’s also been a fairly large number of people diagnosed with mesothelioma, and links occurring to exposure to having done x-rays which contained thorium dioxide. However, thankfully this form of x-rays haven’t been used in years because of the link to cancer.  Those are the majority of known other causes besides asbestos exposure.  Of course, constant research is being done to determine other potential causese of this form of cancer.  Genetic traits, use of substances such as cigarettes, and others still cannot be ruled out.