The common bacterium Chlamydia pneumoniae, known to cause pneumonia and sinus infections, can infect the retina of the eye and the brain. Researchers have found significantly higher levels of this bacterium in the retinas and brains of people with Alzheimer's disease compared to people with normal cognition[1]. The study analyzed retinal tissue from 104 individuals with normal cognition, mild cognitive impairment, or Alzheimer's disease[1]. Higher levels of the bacterium are more common in carriers of the high-risk APOE4 gene and are associated with more severe brain changes and cognitive decline[1]. Infection in human neurons and mouse models of Alzheimer's disease induces inflammation, neuronal cell death, amyloid-beta production and accumulation, and cognitive impairment[1]. The bacterium reaches the retina, where it triggers immune responses associated with inflammation and neurodegeneration, which may serve as a biomarker of risk[1].