Legionella pneumophila is the etiologic agent of Legionnaires disease and which other febrile illness?

• A. Swine fever
• B. Pontiac fever
• C. Rift Valley fever
• D. San Joaquin Valley fever

Answer: (B)

Legionella pneumophila can cause two human illnesses: Legionnaires disease, which is a severe pneumonia, and Pontiac fever, a milder, self-limited febrile illness that resembles a flu-like illness without pneumonia. Pontiac fever results from inhaling aerosolized Legionella and typically presents with fever, malaise, and myalgias, but chest imaging shows no pneumonia. The other options are not linked to Legionella: swine fever is a disease of pigs, Rift Valley fever is a viral zoonosis, and San Joaquin Valley fever is a fungal infection (coccidioidomycosis). Thus, the febrile illness associated with Legionella is Pontiac fever.