The antibody prevalence data indicate that C. pneumoniae infections are common worldwide and more frequently occur in middle-aged and elderly males than in females. C. pneumoniae infections are first acquired in childhood in heavily populated areas, whereas in northern countries, first infections are generally at teenage and in Scandinavia, typically at the time of military service. All chlamydial species tend to cause chronic infections, with severe sequelae developing 10 to 50 years after the primary infections. If C. pneumoniae resides in alveolar macrophages or in vascular endothelial cells in chronic infections the bacteria and their structural components, such as lipopolysaccharides have an easy access to circulation. The continuous induction of cytokines by C. pneumoniae may lead to chronic inflammation of vascular endothelium.