Alzheimer's disease (AD) is a chronic condition in which inflammation has been shown to contribute to neurodegeneration. Current thinking suggests that deposition of beta-amyloid in the brain promotes inflammation resulting in neuronal damage/death. Alternatively, our data suggest that chronic inflammation observed in late-onset sporadic AD may be stimulated by infection with the obligate, intracellular bacterium, Chlamydia pneumoniae. Our results indicate that C. pneumoniae is found in high frequency in glial cells in areas of neuropathology within the brains of patients with AD. Based on our evidence, nervous system infection with C. pneumoniae should be considered a risk factor for sporadic AD.