Pneumocystis jiroveci causes severe pneumonia in which patient population?

• A. Immunocompromised
• B. Immunocompetent
• C. Neonates
• D. Healthy adults

Answer: (A)

Pneumocystis jirovecii is an opportunistic organism that causes severe pneumonia when the host’s cell-mediated immunity is compromised. The immune system—especially T-cell–mediated defense—keeps this organism in check; when immunity is weakened, such as in AIDS or in people receiving immunosuppressive therapy or organ transplants, the organism proliferates in the alveolar spaces and leads to a diffuse, often severe interstitial pneumonia. In individuals with normal immune function, exposure may be asymptomatic or result in only a mild illness, so severe disease is not typical. Therefore, the population at greatest risk for severe Pneumocystis pneumonia is the immunocompromised.