Behaviour is an important way of adapting to disease for individuals and selective pressures resulting from disease have had major consequences for the evolution of behaviour. Behaviour, adrenal and other physiological responses, immunological responses and brain activity all help in coping with disease. Health is an important part of welfare and any pathology implies some degree of poor welfare. Sickness behaviour and physiology are generally adaptive, involve interactions with the immune system and are partly mediated by cytokines. Since, firstly, an understanding of some behaviour requires knowledge of disease and, secondly, responses to disease and studies of brain and behaviour are helping to increase understanding of systems for combating disease, interdisciplinary co-operation is needed for the development of these areas of science.