The hidden history of hypersensitivity pneumonitis.

Hypersensitivity pneumonitis (HP) is a relatively new construct, first reported in the early 20th century, despite major aetiologic factors (farming, bird husbandry) being part of human activities for millennia. Initial confirmed HP reports include exposure to farming and forestry (1932) and bird exposure (1965), much more recently in time than is often assumed. Later changes in occupational and living practices have led to HP associated with isocyanates, machine coolants, indoor mould, hot tubs, and other exposures. Evolution in our pathological understanding of interstitial lung disease in general, wider computerised tomography (CT) utilisation, and advances in immunology and genomics have shaped our modern conceptualisation of HP. Examining historical accounts of HP and its causative factors not only considers when the first cases were recognised, but also explores why the disease emerged at specific times and places, and may provide further insights relevant to the mechanisms underlying HP and disease prevention.