Members of the genus Acinetobacter have been implicated in a wide spectrum of infectious diseases. Although this organism is associated primarily with nosocomial infections, it has also been involved in cases of community-acquired infection. Before the 1970s, Acinetobacter infections were mostly post-surgical urinary tract infections in patients hospitalised in surgical units. The significant improvement in resuscitation techniques during the last 30 years has changed the types of infection caused by Acinetobacter. Since the 1980s, Acinetobacter has spread rapidly among patients in intensive care units. Today, Acinetobacter accounts for c. 9% of nosocomial infections, with most Acinetobacter infections involving the respiratory tract. Transmission via the hands of hospital staff has become the most important contributory factor in patient colonisation. Acinetobacter baumannii is the species that is involved most frequently in infections of humans, but a natural reservoir for A. baumannii outside the hospital environment has not yet been identified. Community-acquired infection and infections acquired following war or natural disasters (e.g., earthquakes) have been described. Acinetobacter causes mild-to-severe illness, but can be fatal. The severity of Acinetobacter infection depends upon the site of infection and the patient's susceptibility to infection as a result of underlying disease. The circumstances that allow Acinetobacter to assume a pathogenic role are not really well-understood. As this organism is a low-grade pathogen, the pathogenesis of Acinetobacter infections probably involves numerous factors, including virulence determinants, which have yet to be investigated.