APOE and Alzheimer disease: For example, in specific susceptible people, such as individuals carrying apolipoprotein E-ε 4 (APOE-ε 4) allele, the blood-brain barrier permeability changes; pathogens such as herpes simplex virus type 1 (HSV-1) can invade the brain and cause chronic inflammation in the brain, which may be related to the occurrence of AD [9].