"C. pneumoniae" is a common cause of pneumonia around the world; it is typically acquired by otherwise-healthy people and is a form of community-acquired pneumonia. Its treatment and diagnosis are different from historically recognized causes, such as "Streptococcus pneumoniae". Because it does not gram stain well, and because "C. pneumoniae" bacteria is very different from the many other bacteria causing pneumonia (in the earlier days, it was even thought to be a virus), the pneumonia caused by "C. pneumoniae" is categorized as an "atypical pneumonia".