A 42-year-old man has had a constant bad taste in his mouth for the past month. On physical examination there are white fluffy patches on the sides of his tongue. These cannot be scraped off. A biopsy is taken and on microscopic examination shows squamous epithelial hyperkeratosis, parakeratosis, and koilocytosis. Immunohistochemical staining for Epstein- Barr virus (EBV) is positive. Which of the following is the most likely risk factor for his oral lesions?