A 29-year-old bird collector presents to the local clinic with what he describes as flu-like symptoms. He doesn't look ill, and has a slight fever, headache, and a dry hacking cough. He denies the production of sputum or hemoptysis. There are no crackles heard on auscultation, and a radiograph shows small streaks of infiltrate. It is determined that he has pneumonia. What is the most likely organism causing his disease

Although all of the organisms listed above cause pneumonia, only two of them are atypical. Atypical pneumonia is characterized by a dry hacking cough, fever, and headache. These include C. psittaci and C. pneumoniae. The mention of birds should point you in the direction of C. psittaci, because they are the reservoir for the organism that is inhaled through dry feces. Typical pneumonias are characterized by hemoptysis of pus-laden sputum, and patients appear very sick.