A 54-year-old working in the sanitation department presents with complaints of an acute onset of SOB. He says that over the past 2 day, he has suddenly become very short of breath. He denies smoking or any other illness. His past medical history is negative. The patient is admitted and requires immediate mechanical ventilation. X-rays reveal an interstitial pattern bilaterally. Two days later, he expires. The pathology only shows fibrosis of the lungs. What had he most likely developed?