On Monday, Tony Gwynn died from complications related to salivary cancer, the same disease that claimed the life of Beastie Boy Adam Yauch two years ago. When Yauch died at the hospital where I work in Manhattan, I was shaken. A whole generation got a little older that day. The thing about Yauch's illness was that nothing immediately jumped out—was it tobacco? Alcohol? Human papillomavirus? Why did he get cancer? No one really knew. But for Gwynn, the answer appeared to be straightforward: The former Padres star believed his cancer was caused by years of using chewing tobacco. But was he right?

Before we dig into cause and effect, it's important to know a bit about facial anatomy. Humans have three main sets salivary glands: the parotid, submandibular, and sublingual glands. (There are also about a thousand minor glands located throughout the mouth, but leave those for another time.) Tony Gwynn reportedly suffered from a malignancy of the parotid gland, which is the most frequent salivary tumor site, accounting for approximately 85 percent of these cancers.

The parotid gland is located next to the ear—the word parotid is derived from paraotic, which literally means around the ear—and patients with salivary cancer typically develop a painless swelling just below the ear, occasionally with facial nerve paralysis like Tony Gwynn had, which carries a very poor prognosis.1

It makes sense that Gwynn attributed his illness to chewing tobacco—the stuff is known to contain toxins that trigger cancer of the lip, tongue and mouth—but as any oncologist will tell you, chewing tobacco hasn't actually been linked to the type of cancer that killed Tony Gwynn.2 Loren Mell, chief of the head and neck radiation medicine service at the Moores Cancer Center in San Diego—and a consulting physician on the team that treated Gwynn—recently weighed in, dismissing Gwynn's assertion that chewing tobacco was to blame. "He may have chewed tobacco," Dr. Mell said, "but that's not likely to be the cause."

So if it wasn't chewing tobacco, what was it? What did Tony Gwynn and Adam Yauch have in common? Why did they both die from the same obscure tumor? It turns out that salivary gland cancer is exceedingly uncommon—only a few thousand cases occur in the United States every year—which makes it difficult to conduct a trial to figure out exactly why it happens. "In the case of parotid cancers," Dr. Mell said, "there's not a single, unified cause that's identified." So while blaming Gwynn's disease on chewing tobacco makes intuitive sense, it's important to know that there isn't any firm medical evidence to actually support the claim.