Simulation games are games for learning based on a reference in the real world. We propose a model for authenticity in this context as a result of a compromise among learning, playing and realism. In the health game used to apply this model, students interact with characters in the game through phone messages, mail messages, SMS and video. Perceived authenticity is measured after the game in 196 phone interviews that yield quantitative and qualitative results. We show evidence of relationships between attributes of the game environment and perceived authenticity. This yields a list of parameters that can be adjusted to favour authenticity. We also study three situations of interaction and show when and why they are perceived as authentic, or not. These results lead to recommendations for the design of simulation games that can be perceived as authentic.