Atypical pneumonia" is an old classification used for respiratory disease that is not lobar and is not "typical." That is, it does not include pneumonia caused by pneumococcus, Klebsiella, Haemophilus, or b-hemolytic streptococci that results in a typical lobular infiltrate. In recent years, the atypical pneumonias have become much more frequent than pneumococcal pneumonia. They are characterized by a slower onset with headache, joint pain, fever, and signs of an acute upper respiratory infection. There are usually no signs of acute respiratory distress, but patients report malaise and fatigue. The most common cause of atypical pneumonia is M. pneumoniae. A quick test for M. pneumoniae infection is cold agglutinins. The test may lack both sensitivity and specificity, but it is rapid and readily available compared with culture of M. pneumoniae or specific antibody formation.In certain age groups (men over 55 years old), Legionnaires disease must be ruled out. While direct microscopy, culture, and serology are available, the detection of Legionella antigen in respiratory secretions is the most sensitive test available.Campylobacter pneumoniae may also cause respiratory infection particularly in, but not limited to, children. Diagnosis is best made by growing these energy-defective bacteria in tissue culture such as HeLa cells. Serology is usually not helpful.During the winter months, Bordetella infection may be quite prevalent, particularly in those patients whose immunizations are not updated. Adult Bordetella infection may not present with typical whooping cough symptoms and must be differentiated from other forms of acute bronchitis by culture on specific media or direct fluorescent microscopy.Direct microscopy of sputum is an important preliminary test for sputum and other exudate specimens. One needs to be aware that the information obtained is limited in interpretation. No specific identity of a microorganism can be determined by gram staining and even mixtures of bacteria can be inferred. On the other hand, examination of gram stains may provide enough information for the physician to make immediate decisions on treatment and other management that may be modified by future laboratory results. Fluorescent Ab detection of an organism in sputum is more specific than gram staining. Here specific known Abs (single or in a mixture) are used to identify organisms present.