Chlamydia pneumoniae is a common respiratory bacterium responsible for millions of cases of pneumonia and sinus infections each year. Most people recover without incident, but a growing body of research suggests the bacterium doesn’t always leave. In a February 2026 study, published in Nature Communications, researchers at Cedars-Sinai found that Chlamydia pneumoniae can linger in the eye and brain for years, potentially aggravating Alzheimer’s disease. Scientists found that greater amounts of the bacterium were associated with more severe brain damage and worse cognitive decline, with elevated bacterial levels especially common in individuals carrying the APOE4 gene variant.

The mechanism is alarming. The bacterium can infect the olfactory and trigeminal nerves and reach the brain within 72 hours, while also dysregulating key pathways involved in Alzheimer’s disease pathogenesis.

The federal government has taken notice. The Senate Appropriations Committee’s FY 2026 spending bill proposed an increase of $100 million for Alzheimer’s disease and related dementias research at NIH National Institute on Aging. Researchers hope this increased funding hope will accelerate investigation into infection-driven neurodegeneration. What once seemed like an ordinary respiratory bug may prove to be one of the brain’s most dangerous long-term adversaries.