PLE-1906

A sexually active 24-year-old woman, known to be HIV-1 infected, has had a fever for 2 days and a productive cough. Chest radiographs show an infiltrate in the right lung. Two weeks earlier, her helper T cell count was 510/mm3. Testing for nontreponemal antigen (rapid plasma reagin) is positive at two dilutions, and treponemal antigen testing is also positive. What is the most likely cause of the pneumonia?