a 14% lower risk of lung cancer in the U.S. study. Similar results were obtained among individuals who were smokers. Due to tobacco The most important chemical compounds in smoked tobacco that are carcinogenic are those that produce DNA damage since such damage appears to be the primary underlying cause of cancer. Cunningham et al. combined the microgram weight of the compound in the smoke of one cigarette with the known genotoxic effect per microgram to identify the most carcinogenic compounds in cigarette smoke. These compounds and their genotoxic effects are listed in the article Cigarette. The top three compounds are acrolein, formaldehyde and acrylonitrile, all known carcinogens. Due to infection Viruses In 2002 the World Health Organizations International Agency for Research on Cancer estimated that 11.9% of human cancers are caused by one of seven viruses (see Oncovirus overview table). These are Epstein-Barr virus (EBV or HHV4); Kaposi's sarcoma-associated herpesvirus (KSHV or HHV8); Hepatitis B and Hepatitis C viruses (HBV and HCV); Human T-lymphotrophic virus 1 (HTLV-1); Merkel cell polyomavirus (MCPyV); and a group of alpha Human papillomaviruses (HPVs). Bacteria Helicobacter pylori and gastric cancer In 1995 epidemiologic evidence indicated that Helicobacter pylori infection increases the risk for gastric carcinoma. More recently, experimental evidence showed that infection with Helicobacter pylori cagA-positive bacterial strains results in severe degrees of inflammation and oxidative DNA damage, leading to progression to gastric cancer. Other bacterial roles in carcinogenesis Perera et al. referred to a number of articles pointing to roles of bacteria in other cancers. They pointed to single studies on the role of Chlamydia trachomatis in cervical cancer, Salmonella typhi in gallbladder cancer, and both Bacteroides fragilis and Fusobacterium nucleatum in colon cancer. Meurman has recently summarized evidence connecting oral microbiota with carcinogenesis. Although suggestive, these studies need further confirmation. Common underlying factors in cancer Mutations One underlying commonality in cancers is genetic mutation, acquired either by inheritance, or, more commonly, by mutations in one's somatic DNA over time. The mutations considered important in cancers are those that alter protein coding genes (the exome). As Vogelstein et al. point out, a typical tumor contains two to eight exome "driver gene" mutations, and a larger number of exome mutations that are "passengers" that confer no selective growth advantage. Cancers also generally have genome instability, that includes a high frequency of mutations in the noncoding DNA that makes up about 98% of the human