This patient likely has Legionella pneumophila infection, a common cause of community-acquired pneumonia (CAP).  Most cases present with high fever and fatigue followed by gastrointestinal (eg, nausea, vomiting, diarrhea), pulmonary (eg, cough, shortness of breath), and nonspecific (eg, headache, confusion) symptoms.  Patients age >50 who smoke and have chronic lung disease are at greatest risk.  Isolated cases are common, but outbreaks can occur when the pathogen contaminates manmade water reservoirs (eg, hot tub, air cooling system, drinking water system).

L pneumophila is a facultative intracellular organism that requires specialized media (eg, buffered charcoal yeast extract) to grow; although the pathogen is gram negative, it stains poorly with Gram stain due to a unique lipopolysaccharide in its outer membrane.  Therefore, the diagnosis is most commonly confirmed based on positive PCR testing or positive urine Legionella antigen testing.

(Choice A)  Coccidioides immitis is a dimorphic fungus endemic to the deserts of the southwestern United States.  Although most cases of C immitis infection are asymptomatic, it can cause CAP.  However, the presence of gastrointestinal symptoms, headache, and confusion makes Legionella far more likely.

(Choice B)  Klebsiella pneumoniae is a gram-negative rod that causes nosocomial pneumonia and aspiration pneumonia.  However, this organism can generally be visualized on sputum Gram stain and does not typically cause concurrent diarrhea.

(Choice D)  Mycobacterium kansasii is an atypical mycobacterium that causes an illness similar to Mycobacterium tuberculosis.  However, diarrhea, confusion, and high fever are relatively uncommon.

(Choice E)  Mycoplasma pneumoniae infection causes tracheobronchitis or walking pneumonia.  M pneumoniae cannot be detected on Gram stain due to a lack of peptidoglycan in the cell wall.  However, symptoms of infection are typically subacute and mild.  The presence of high fever and confusion makes an alternate diagnosis far more likely.

Educational objective:
Legionella pneumophila infection is typically marked by high fever and fatigue followed by significant gastrointestinal, pulmonary, and constitutional symptoms.  The organism is not well visualized on Gram stain and does not grow on traditional culture media; L pneumophila infection is primarily diagnosed by polymerase chain reaction or urine antigen testing.