[2] The most common symptoms of acute interstitial pneumonitis are highly productive cough with expectoration of thick mucus, fever, and difficulties breathing.

[citation needed] Acute interstitial pneumonitis typically progresses rapidly, with hospitalization and mechanical ventilation often required only days to weeks after initial symptoms of cough, fever, and difficulties breathing develop.

[citation needed] Rapid progression from initial symptoms to respiratory failure is a key feature.

[citation needed] Sixty percent of people with acute interstitial pneumonitis will die in the first six months of illness.

[citation needed] Acute interstitial pneumonitis occurs most frequently among people older than forty years old.

[citation needed] Acute interstitial pneumonitis was first described in 1935 by Louis Hamman and Arnold Rich, and given the name Hamman–Rich syndrome.