[Incident-crisis-disaster. Conceptualizing unusual biological incidents].

Unusual biological threats demand adequate preparedness efforts, as demonstrated, for example, by the Ebola virus disease outbreak in West Africa in 2014/2015 and pandemic influenza in 2009/2010. In Germany, responsibilities for such efforts are located in different governmental authorities and differ from state to state. As a result, there are many different preparedness approaches using divergent core terminology.In this article a common definition for the term "unusual biological incident" is proposed. To do so, a literature review as well as semi-structured expert interviews with representatives of central actors in Germany were conducted. The understanding of "unusual biological incident" was not consistent among experts; four approaches to qualify a biological incident as "unusual" were identified. These were merged in a comprehensive system-oriented approach that focuses on the health system's resilience and on shortages of knowledge and material resources during incidents.Based on this approach, we suggest a stage model for the categorization of biological threats as "incident," "crisis," "severe crisis," or "disaster." The need for central coordination is a defining characteristic to qualify a biological incident as "unusual." Based on the identified shortages, the necessary response strategies can be derived.