A 16-year-old male from rural Arizona presents with 3 days of fever, myalgias, and headache. He now develops acute dyspnea. Vital signs: temperature 38.7°C (101.66°F), heart rate 115 bpm, respiratory rate 30/min, blood pressure 85/50 mmHg, oxygen saturation 88% on room air. Labs show WBC 19,000/μL with immunoblasts, hematocrit 54%, platelets 58,000/μL. Chest X-ray reveals bilateral interstitial infiltrates. Which virus is most likely responsible?