Terrorism is a very sad reality for countries all over the world. Our society and its infrastructures are not prepared to face such threat. Most of buildings receiving people, are open spaces and can be easily attacked by terrorists, like in Paris on November 13, 2015. One of the vulnerable places is the hospital which is easily accessible by any people, in order to facilitate the health care. We propose a vulnerability assessment approach to increase the resilience of hospitals. Our quantitative approach first evaluates the likelihood of the threats taking into account on one hand the motivation and capabilities of terrorists, and on the other hand the accessibility of potential targets that the critical assets define. These latter are identified by building an IDEFØ model of the hospital patient flows. Second, the impacts of terrorist scenarios on the critical assets, are evaluated by a linear program which is a direct translation of the IDEFØ model to a flow model. The impacts of the worst scenarios are reduced, by proposing effective countermeasures which are evaluated by this flow model.