The COVID-19 pandemic profoundly affected children’s everyday experiences in schools worldwide. As future outbreaks are inevitable, it is essential to understand how teaching and learning about infectious diseases was shaped by the COVID-19 pandemic, in order to develop biology education to meet the needs for future epidemic-prone diseases. This thesis investigates how knowledge about infectious diseases is transformed from sources external to the school system into knowledge presented and understood in Swedish middle school classrooms. The overarching research question is: How is the portrayal of knowledge about infectious diseases reflected in biology textbooks, teachers’ practices, and pupils’ understanding in the aftermath of COVID-19? The thesis comprises four empirical studies, each examining a distinct stage in this transformation of knowledge, from academic and societal domains to what is actually taught and learned. The findings are interpreted through contagion literacy, a framework outlining the competencies expected of health-literate citizens and related to the concept of powerful knowledge. A comparative textbook analysis (Paper I) of editions published before and after the pandemic reveals that representations of infectious diseases remained largely unchanged, with an emphasis on functional health literacy describing content knowledge rather than how to act upon this knowledge. In practice, teaching during the pandemic was shaped by pupil-initiated, question-driven discussions about current events, rather than by an expansion of the planned biology curriculum (Paper II). Studies of pupils’ understanding (Papers III and IV) show that while COVID-19 influenced their thinking, their conceptual grasp of infectious diseases and microorganisms remained limited and often based on informal sources such as the media and the internet, rather than formal biology education. The thesis argues for the deliberate integration of contagion literacy into science education with the addition of the host and its immune response as new aspects. Such knowledge constitutes potential powerful knowledge, essential for preparing pupils to navigate future public health challenges and pandemics as critically engaged citizens