According to the BPS website "Health psychology is a new and rapidly evolving area, and can be defined as the practice and application of psychological methods to the study of behaviour relevant to health, illness and health care. For example, the study of why and when people seek professional advice about their health, why they do or do not recommend preventative measures, how patients and health care professionals interact, how patients adapt to illness, and the links between perception, health behaviour and physical functioning. "

Though a relatively new field huge developments have been taking place and their appears to be a great many jobs in this area for appropriately accredited professionals. There are currently 18 institutions across the UK running accredited Masters level courses.

You can find further information at the Division of Health Psychology Website

Recommended reading:

 They F*** You Up ~ Oliver James

 Predicting Health Behaviour ~ Mark Conner (Editor), Paul Norman (Editor)

 The Social Context of Health ~ Michael Hardey

  Health Psychology ~Jane Ogden

  Health Behaviour Change ~Stephen Rollnick, et al

  Women and Self-harm ~Gerrilyn Smith, et al