PURPOSE OF REVIEW: To review the role of microorganisms in interstitial lung disease (ILD) and to emphasize their importance in initiation and course of ILD. RECENT FINDINGS: ILD can be idiopathic but often causality such as drugs or connective tissue disease can be found. Multiple microorganisms have been associated with ILD. On the one hand, pulmonary infection can cause extensive pulmonary damage with patterns of an ILD. On the other hand, microorganisms can trigger the immune system and provoke an abnormal response- not directed against the causative pathogen- that may result in ILD. Moreover, patients with ILD often are susceptible to infection, and infections can importantly influence the course of ILD. Furthermore, not only an infection but also its treatment can result in a drug-induced pneumonitis, eventually resulting in long-term lung damage. SUMMARY: Microorganisms can initiate and/or influence the course of ILD. Early recognition, adequate diagnostic evaluation and therapy are essential to prevent permanent damage. Prevention of infection in patients with established ILD is strongly recommended.