Background To our knowledge, there are few indices that can assess oral health in the elderly. Those that are currently used are subjective and test the impact of oral health on quality of life. It appears to be relevant to create an easy and reproducible index, based on an oral health clinical exam that can be correlated with infectious disease risk in the elderly. Objectives This article aims to explain the methodology used to create an oral health index to assess infectious disease risk in elderly patients more than 65 years old. Design Cluster randomized trial. Setting Long-term care and rehabilitation facilities. Participants All patients who were included were 65 years old or older and were present in participant wards at the beginning of the study or were admitted during the study period. Measurements The assessment index was developed with the assistance of two dentists, after conducting a literature review. This was an index composed of eight criteria, which included most oral health problems associated with infections in the elderly. A score of 1 was given if a single criterion was observed. The final score ranged from 0 to 8 (0 for a healthy oral health index and 8 for a bad index). Results The oral health index was used in 2513 patients. This index grew significantly with age (P = 0.002) and malnutrition (P = 0.004). The pneumonia incidence rate was significantly correlated with the oral health index (P = 0.002). Conclusion This index is easy to use, with few items, and it takes into account most infectious disease risk factors. It would be interesting to integrate this score into a routine admission check-up process in the elderly for the purpose of preventing infectious diseases.