Background. As demonstrated during the global Ebola crisis of 2014-2016, healthcare institutions in high resource settings need support concerning preparedness during threats of infectious disease outbreaks. This study aimed to exploratively develop a standardized preparedness system to use during unfolding threats of severe infectious diseases. Methods. A qualitative three-step study among infectious disease prevention and control experts was performed. First, interviews (n=5) were conducted to identify which factors trigger preparedness activities during an unfolding threat. Second, these triggers informed the design of a phased preparedness system which was tested in a focus group discussion (n=11). Here preparedness activities per phase and per healthcare institution were identified. Third, the preparedness system was completed and verified in individual interviews (n=3). Interviews and the focus group were recorded, transcribed, and coded for emerging themes by two researchers independently. Data were analyzed using content analysis. Results. Four preparedness phases were identified: Preparedness phase green is a situation without the presence of the infectious disease threat that requires centralized care, anywhere in the world. Phase yellow is an outbreak in the world with some likelihood of imported cases. Phase orange is a realistic chance of an unexpected case within the country, or unrest developing among population or staff; phase red is cases admitted to hospitals in the country, potentially causing a shortage of resources. Specific preparedness activities included infection prevention, diagnostics, patient care, staff, and communication. Consensus was reached on the need for the development of a preparedness system and national coordination during threats. Conclusions. In this study, we developed a standardized system to support institutional preparedness during an increasing threat. Use of this system by both curative healthcare institutions and the (municipal) public health service, could help to effectively communicate and align preparedness activities during future threats of severe infectious diseases.