APOE and Alzheimer disease: Notably, several AD risk-associated genes including APP, APOE, and TREM2, have been associated with immune activation, and a number of bacterial pathogens (e.g., H. pylori, C. pneumoniae, and Porphyromonas gingivalis) have been detected in the brains of AD patients (Price et al., 2006; Harris and Harris, 2015; Miklossy, 2015; Pisa et al., 2015; Carter, 2017).