Health psychology is concerned with understanding human behaviour in the context of health and illness. Health psychology emerged as a separate discipline in the 1970s, and there are many reasons for this and for its rapid development. A wide range of behaviours can influence health. A major problem for health psychologists is to understand the way in which stress is associated with the development of illness. Personality variables can influence health in a variety of ways, and a broad distinction can be made between those with a positive relation to health and health behaviour and those with an adverse relation. One very obvious way in which behaviour can influence health comes from research on lifestyle, which has identified a number of behaviours that can have both positive and negative effects on health status. For health psychologists, the key questions about health-risk behaviours concern their origin, their maintenance, and their prevention or treatment.