The intracellular bacterium Chlamydophila ("Chlamydia") pneumoniae is a pathogen for several respiratory diseases and may be a factor in the pathogenesis of chronic diseases of aging including atherosclerosis and Alzheimer's disease. We assessed whether aging is coupled with increased burden of infection in BALB/c mice after intranasal infection by C. pneumoniae. Six-and twenty-month-old BALB/c mice were infected intranasally with 5 ؋ 10 4 inclusion forming units (IFU) or 5 ؋ 10 5 IFU of C. pneumoniae. Lung, brain, and heart tissue were analyzed for infectious C. pneumoniae and for Chlamydophila antigen by immunohistochemistry. At both doses, aging was associated with a decreased proportion of animals that cleared infection from the lung and greater burden of infectious organism within the lung. We observed dose-dependent spread to the heart/ascending aorta in animals infected with C. pneumoniae. In mice given 5 ؋ 10 4 IFU, spread to the heart by day 14 was only observed in old mice. By day 28, all animals inoculated with 5 ؋ 10 4 IFU showed evidence of spread to the heart, although higher C. pneumoniae titers were observed in the hearts from old mice. In mice inoculated with 5 ؋ 10 5 IFU, spread of C. pneumoniae to the heart was evident by day 14, with no discernible age effect. C. pneumoniae was also recovered from the central nervous system (brain and olfactory bulb) of all mice by day 28 postinfection, with higher C. pneumoniae titers in old animals than in young animals. Our results suggest that infection with C. pneumoniae may be more severe in old animals.With the life expectancy increasing over this century and predicted to continue to rise over the course of the next half century, it is expected that, by the middle of the 21st century, more than 20% of the population will be over the age of 65 (19).

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