The technology uses aggregated health data and outbreak models to conduct differential diagnosis and provide risk assessments that indicate a likelihood of contracting COVID-19. A healthcare organization server provides COVID-19 diagnostic kits and health applications to participating computing devices that log user health data and registers contacts with other participating users via wireless interactions. The organization compares received data with data received from computing devices from a plurality of other users and identifies common occurrences. The server creates an outbreak model of a geographic region based on the data. The server communicates a determined likelihood of contracting COVID-19 of the user to the user computing device. The server provides the outbreak model to the user, healthcare workers, or others for use in responding to the outbreak.