A previously healthy 24-year-old man presented with a 2-day history of fever, myalgias, nausea, and vomiting. On the morning of hospital admission, cough and progressive dyspnea developed in the patient. He denied other symptoms, including chest pain, abdominal pain, or sore throat. This military recruit had been participating in field training in southern New Mexico for 2 weeks prior to presentation and had previously been living in Maricopa County, AZ. He lived in field barracks but denied sick contacts, and no other members of his unit were ill. He reported no contact with household or farm animals, rabbits, birds, or toxins, and he denied the use of illicit drugs.