Objectives To describe and explain peoples' developing threat appraisal and representations of the novel illness COVID-19 over the first months of the pandemic. The Common-Sense Model of illness perceptions provided the theoretical framework. Design A cross-sectional study with 511 respondents and a follow-up study 4 months later on 422 respondents completing an online survey measuring demographic factors, media consumption, self-assessed health, experience with the disease, health anxiety, COVID-19 threat, worries and cognitive and emotional illness representations. Results Health anxiety, media consumption, female gender, lower self-assessed health, knowing a deceased COVID-19 patient and being infected explained variance in threat appraisal. Worries represented 2 factors: psychosocial and existential. Threat appraisal and worries explained variance in illness representations. Representations of the disease worsened and started stabilizing over time. Emotional representations were exceptionally stable and explainable by threat appraisals. Conclusions These studies revealed the initial stages of developing representations of a new disease in the context of the COVID-19 pandemic. Gaining insights into those representations is key to understanding, predicting and modifying behavioral and mental responses to the pandemic.