Rationale- Relationships between chronic health and acute infections remain poorly understood. Preclinical studies suggest cross talk between nervous and immune systems. Objectives- To determine bidirectional relationships between cognition and pneumonia. Methods- We conducted longitudinal analyses of a population-based cohort over ten years. We determined whether changes in cognition increase risk of pneumonia hospitalization by trajectory analyses and joint modeling. We then determined whether pneumonia hospitalization increased risk of dementia using a Cox model with pneumonia as a time-varying covariate. Results- Of the 5,888 participants, 639 (10.9%) were hospitalized with pneumonia. Most had normal cognition before pneumonia. Three cognition trajectories were identified: no, minimal, and severe rapid decline. A greater proportion of participants with pneumonia were on trajectories of minimal or severe decline compared to those never hospitalized with pneumonia (proportion with no, minimal and severe decline were 67.1%, 22.8%, and 10.0% vs. 76∙0%, 19.3%, and 4.6% for participants with and without pneumonia, respectively, P<0.001). Small changes in cognition increased risk of pneumonia, even in well appearing participants (β= -0.02, P<0∙001). Participants with pneumonia were subsequently at an increased risk of dementia (hazard ratio=2.24 (95% CI 1.62-3.11), P=0∙01). Associations were independent of demographics, health behaviors, chronic conditions, and physical function. Bidirectional relationship did not vary based on severity of disease, and similar associations were noted for those with severe sepsis and other infections. Conclusions- A bidirectional relationship exists between pneumonia and cognition and may explain how a single episode of infection in well-appearing older individuals accelerates decline in chronic health conditions and loss of functional independence.