Memory bias, the tendency to rely on certain events over others, can become an issue in chronic illnesses, especially when symptoms are reported retrospectively. This paper examines a case where continuous symptom registration can be facilitated, memory supported, and memory bias reduced by introducing a mobile application. The aim of the paper is to report on the design of an app for collecting subjective data over an extended period to continuously follow children with periodic fever. The research approach is qualitative, building on interview data. The design method is co-design, a collaborative and participatory approach involving researchers, physicians and other key stakeholders, with focus on the views of the parents. We argue that collecting data continuously through an app moves the discussion from memory to the specific data points, which is illustrated through trends shown in the visualizations of the data. Moreover, we highlight the importance of systematically collecting data over an extended period through a data-driven approach to both forward clinical practice and research on complex, often chronic topics such as periodic fever, which is genuinely under-researched to date.