The numbers that I have seen are that 25% of H&N cancer patients NEVER drank or smoked so where the doctor got that number is beyond me (maybe that's in his personal experience). Maybe Brian has something to share on this. HPV has fairly recently been cited as a causitive factor as well.

Straight from the NCI:

"Oral Cancer: Who's at Risk?

Doctors cannot always explain why one person develops oral cancer and another does not. However, we do know that this disease is not contagious. You cannot "catch" oral cancer from another person.

Research has shown that people with certain risk factors are more likely than others to develop oral cancer. A risk factor is anything that increases your chance of developing a disease.

The following are risk factors for oral cancer:

Tobacco: Tobacco use accounts for most oral cancers. Smoking cigarettes, cigars, or pipes; using chewing tobacco; and dipping snuff are all linked to oral cancer. The use of other tobacco products (such as bidis and kreteks) may also increase the risk of oral cancer. Heavy smokers who use tobacco for a long time are most at risk. The risk is even higher for tobacco users who drink alcohol heavily. In fact, three out of four oral cancers occur in people who use alcohol, tobacco, or both alcohol and tobacco.
Alcohol: People who drink alcohol are more likely to develop oral cancer than people who don't drink. The risk increases with the amount of alcohol that a person consumes. The risk increases even more if the person both drinks alcohol and uses tobacco."

You did notice the "3 out of 4"...