We propose an observation and formalization method to describe individual and collective activities performed by a medical team. The goal of this paper is to introduce this method (eorca) and to illustrate its application in the case of a medically complex, uncertain and time-constrained situation: the management of an outbreak early warning by epidemiology specialists. This method comprises two main steps: the formal observation of medical staff's activities that occur during medical situation management and a representation of the findings with regard to an ontology and a temporal flowchart, which describes actors and events related to patient management. The application of this method in the case of this situation is based upon collaborative activities. It shows that the management of an epidemiological early warning is characterised by collective activity. Numerous information transmissions are required between members of the team. Its goal is to identify diseases and to prepare adequate actions in a context of uncertain events and in a short interval of time. Elements of cooperative activities between actors were outlined in our observations: namely, the actors jointly develop a set of structured and complex processes and the allocation of tasks among the actors (e.g., parallel or successive actions) depends on the complexity and urgency of the situation. We have observed that tasks were jointly performed by a set of caregivers sharing the same objectives and were based on a mutual and distributed understanding of intentions and actions. The results of this study illustrate the setting up of a shared problem representation, implying a strong element of cooperative work in a decision-making process. In this context, multiple and possibly divergent interpretations of problems may arise and co-exist. This is especially observed when actors are faced with a problem that is characterized by a high level of complexity and/or lack of accurate data, or do not have access to the relevant knowledge needed to fully understand the problem.