Gene has gone to his doctor because he has developed intense coughing and difficulty breathing. He also has fever, diarrhea, and night sweats. The patient history reveals that he lives a fairly ordinary life since recovering from drug addiction about six years ago. Before that time, Gene habitually injected "recreational drugs," often sharing his equipment with his friends. He has been sick now and then, as he puts it, "with the ordinary colds and flu," but he does not remember any specific, serious illness. Examination reveals swollen lymph nodes, and his weight is lower than at his last visit. The doctor orders a staining test on the material Gene is coughing up, and takes a blood sample. The sputum test is positive for Pneumocystis carinii.