A 54-year-old working in the sanitation department presents with complaints of an acute onset of shortness of breath. He says that over the past two days, 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 shows fibrosis of the lungs and hyaline membranes. What had he most likely developed?