ABSTRACT
Pneumocystis pneumonia (PCP) is an opportunistic fungal infection associated with human immunodeficiency virus (HIV) infection as an acquired immunodeficiency syndrome (AIDS)-defining illness. The PCP incidence in patients with HIV has declined over the last few decades due to effective antiretroviral therapy and prophylaxis. The PCP incidence in HIV-negative patients has increased due to the increasing use of a wide array of immunosuppressants in cancer and autoimmune disease. PCP clinical course varies from patients with HIV in their clinical features, the severity of clinical presentation, and mortality. PCP in autoimmune diseases is rare, especially in rheumatoid arthritis (RA) in the United States of America (USA). Here, we describe an elderly Caucasian female with rheumatoid arthritis and left lung mucinous adenocarcinoma status post recent resection with no chemotherapy on a low dose of methotrexate (MTX) and prednisone presenting with acute hypoxic respiratory failure due to PCP from absolute lymphopenia.