ABSTRACT
Bacterial agents causing so-called atypical pneumonias are usually considered to include Mycoplasma pneumoniae, Chlamydia (Chlamydophila) spp., and Legionella spp. In recent years, the two most common agents in human respiratory infections, M. pneumoniae and C. pneumoniae, have also been associated with asthma. They are both capable of causing frequent reinfections. Both pathogens are extremely common and no one seems to avoid contact with them. The outcome of infection is mainly dependent on the host response. Persistent infections are typical for chlamydiae, and the possibility of chronic mycoplasmal infections has recently also received attention. In these chronic infections, the activated immune defence mechanisms are evidently mainly responsible for the damage seen.
