But none of these factors help to explain Yauch's diagnosis.

"Usually, if you've been treated with radiation for another disease, that can be a factor," said Genden.  "But he was never treated with radiation for anything else, to my knowledge.  The other potential cause is environmental exposure, but that's typically for wood workers or people who work with nickel - which also doesn't apply to him."

According to Genden, the development of malignant salivary gland cancer is more a result of bad genes than bad habit.

"Salivary gland cancer usually occurs as a result of a sporadic genetic defect," said Genden.  "So in other words, something in the genes goes wrong and the cancer develops."

The most common treatment for salivary gland cancer is surgery, sometimes followed by radiation treatments to stop the cancer from spreading.  Yauch underwent both the surgery and radiation therapy after his diagnosis in 2009.