Malaria is endemic in Brazilian Amazonia, accounting for 99% of national cases. Amazonian forest peoples (both Indigenous and traditional) understand and treat the disease based on their knowledge, rituals, and religion. In recent decades, biomedical health coverage has expanded in the region, with implications for local perceptions and practices to prevent, treat, and recover from malaria. This paper attempts to understand how the expansion of biomedical healthcare among forest peoples interacts with their ethnomedicinal knowledge. Our results clearly indicate that most of our research participants in rural northwest Amazonia believe that malaria has a variety of causes, forms of prevention, and treatment. We also found that these beliefs are shaped by both local knowledge (including Indigenous) and some technical concepts of biomedicine. Consequently, new approaches and practices in healthcare need to be developed which consider forest peoples' perceptions and understanding.