Although exposure to asbestos is the primary cause of mesothelioma, a slow-developing cancer, researchers believe that infection by the SV40 virus is a contributing cause, since in the United States 60 percent of tissue samples from mesotheliomas, but none from healthy tissue, contain SV40  SV40 is a monkey virus; however, in 1960 some polio vaccine was contaminated with the virus
Researchers hypothesize that this vaccine was the source of the virus found in mesotheliomas decades later
Recently discovered samples of the vaccine dating from 1960 still show traces of the virus.