A 21-year-old college student complained of malaise, low-grade fever, and a harsh cough, but not of muscle aches and pains. An x-ray revealed a diffuse interstitial pneumonia in the left lobes of the lung. The WBC count was normal. The student has been ill for a week. Based on the information given, the most likely diagnosis is

Mycoplasma pneumoniae causes a respiratory infection known as primary atypical pneumonia or walking pneumonia. Although disease caused by M. pneumoniae can be contracted year-round, thousands of cases occur during the winter months in all age groups. The disease, if untreated, will persist for 2 weeks or longer. Rare but serious side effects include cardiomyopathies and central nervous system complications. Infection with M. pneumoniae may be treated with either erythromycin or tetracycline. The organism lacks a cell wall and so is resistant to the penicillin and the cephalosporin groups of antibiotics. Until recently, diagnostic tests have been of limited value. Up to 50% of cases may not show cold agglutinins, an insensitive and nonspecific acute-phase reactant. However, if cold agglutinins are present, a quick diagnosis can be made if signs and symptoms are characteristic. Complement fixation tests that measure an antibody to a glycolipid antigen of M. pneumoniae are useful but not routinely performed in most laboratories. Also, cross-reactions may occur. Culture of M. pneumoniae, while not technically difficult, may take up to 2 weeks before visible growth is observed. A DNA probe is available. It is an 125I probe for the 16S ribosomal RNA of M. pneumoniae. Evaluations in a number of laboratories indicate that compared with culture it is highly sensitive and specific. Reference: Ananthanarayan and Paniker&;s Textbook of Microbiology Tenth edition