Atypical pneumonia, also known as walking pneumonia, is the type of pneumonia not caused by one of the more traditional pathogens. Its clinical presentation contrasts to that of "typical" pneumonia. A variety of microorganisms can cause it. When it develops independently from another disease it is called primary atypical pneumonia (PAP). Distinction between atypical and typical pneumonia is, however, medically insufficient. For the treatment of pneumonia, it is important to know the exact causal organism.