To explain the association between AD and the H. pylori infection, it was proposed that the access of H. pylori to the brain might occur by (1) an oral-nasal-olfactory pathway; (2) monocytes infected with this bacterium that produce higher levels of TNF, leading to a disruption in BBB; and (3) a rapid retrograde neural pathway from the gastrointestinal tract [66]. The gene discussed is TNF; the disease is Alzheimer disease.