There are two types of natural history studies: prospective and retrospective. A natural history study examines a group of people who have a specific medical condition or disease or are at risk of developing one. A natural history is non-interventional and collects health information in order to understand how the medical condition or disease develops and how to possibly treat it. No treatment or investigational product is given. In a prospective study, this data collection is forward-looking and is done over time in the future. In a retrospective study, researchers review and examine factors related to an outcome in the past by looking back on past exposures and medical events.