**4. Atypical pneumonia**

Atypical pneumonia refers to pneumonia caused by atypical bacteria, including *Legionella* species, *Chlamydia pneumoniae* and *Mycoplasma pneumoniae* [29]. It is called "atypical" because the symptoms and signs differ from those of pneumonia due to other common organisms. It is generally regarded that *M. pneumoniae*, *Legionella* spp., *C. pneumoniae, Chlamydophila psittaci and Coxiella burnetii* are the key CAP pathogens not readily identified by standard culture methods [30]. Other atypical pathogens include viruses, atypical mycobacteria, *Francisella tularensis* and an extensive list of agents of bioterrorism.
