Most mouth cancer is oral squamous cell carcinoma. This is uncommon in the developed world, except in parts of France, but is common in the developing world, particularly South East Asia and Brazil. It is mainly seen in men over middle age (though it is increasing in younger people), tobacco users, and lower socioeconomic groups. Aetiological factors (acting on a genetically susceptible individual) include tobacco use (75% of people with oral cancer smoke), betel use (Bidi leaf, and often tobacco, plus spices, slaked lime, and areca nut), alcohol consumption, a diet poor in fresh fruit and vegetables, infective agents (Candida, viruses), immune deficiency, and (in the case of lip carcinoma) exposure to sunlight. Additional primary neoplasms may arise mainly in the aerodigestive tract. This occurs in up to 25% of people who have had oral cancer for over three years, and in up to 40% of those who continue to smoke. Similarly, patients with lung cancer are at risk from second primary oral cancers. Potentially malignant lesions or conditions may include some erythroplasias, dysplastic leucoplakias (about half of oral carcinomas have associated leucoplakia), lichen planus, submucous fibrosis, and chronic immunosuppression. Rare causes of oral cancer include tertiary syphilis, discoid lupus erythematosus, dyskeratosis congenita, and Plummer-Vinson syndrome (iron deficiency and dysphagia).