What happens when new health information is introduced into a community? We have explored this question in a semi-rural community of Southeastern Tanzania whose population has been in contact with biomedicine for many decades. With the example of malaria, we illustrate how biomedical knowledge transmitted in health messages coexists, interacts and merges with local pre-existing ideas and logics. The results are syncretic models, which may deviate considerably from what health promoters intended to transmit. Some of those may have implications for treatment of malaria, which may include delay in seeking treatment and non-compliance with therapy. Analysing this medical syncretism clearly demonstrates that even if comprehension of health messages is accurate, the way in which people interpret these messages may not be. Disentangling syncretic processes permits us to understand the dynamics of how information is processed by the recipients, and provides orientations for health promoters for adapting messages to the local context.