Atypical pneumonia is pneumonia that is not caused by one of the traditional microorganisms, such as Streptococcus pneumoniae. Once atypical pneumonia is diagnosed, it is important to identify the causative organism so treatment can be tailored to the pathogen. This issue covers specific tests for atypical pathogens as well as taking an in depth look at specific microorganisms and diseases, including mycoplasma, legionella, and SARS.