A 58-year-old woman presented to the office of her primary care physician with a history of cough and dyspnea on exertion for 2 weeks. Her medical history included diabetes, hypertension, and hyperlipidemia. She smoked one pack of cigarettes a day and had a 40 pack-year history of tobacco use. She complained of chronic dyspnea on exertion, which had worsened over the past 2 weeks. She also complained of nonproductive cough during this 2-week period. Over the past 2 years, she had experienced intermittent night sweats but no fevers. Her weight had remained stable. She denied any recent travel. At the office of her physician, she was found to have an oxygen saturation of 85% on room air, and she was subsequently admitted to the hospital for further evaluation.