Abstract: Objective To discuss the relationship between herpes simplex virus type 1 (HSV-1) infection and Alzheimer disease (AD) in elderly patients. Methods A total of 101 AD patients (74 men and 27 women aging from 77 to 93) were included. Meanwhile, another 107 healthy subjects (81 men and 26 women aging from 71 to 89) were selected as a control group. Their cognitive functions were assessed according to the Alzheimer Disease Assessment Scale-Cognitive 12 (Cog-12), the Montreal Cognitive Assessment (MoCA), and the Mini Mental Status Examination (MMSE). Antibody to HSV-1 and plasma tau protein were detected by ELISA. Results There was no statistical difference as to age and gender between the two groups. Group AD showed a higher positive rate of antibody to HSV-1 than the control group (79.2% vs. 58.0%, P<0.05). According to logistic regression analysis, antibody to HSV-1, diabetes, and hyperlipidemia were risk factors for the development of AD. Conclusions HSV-1 infection is a risk factor to Alzheimer disease. There was no relationship between the level of tau protein in the peripheral blood plasma and AD.