Disease is best understood as the disturbance of homeostasis, i.e. it impairs the function of an individual in some way, usually producing signs and symptoms. Human health is defined by the World Health organisation as a ‘state of complete physical, mental and social wellbeing and not merely the absence of disease.’ The problem with defining health and disease is that there are instances where an individual can appear healthy but be suffering from a disease. For example, a person who carries an infectious pathogen such as HIV may be ‘healthy’ if the symptoms of the disease AIDS have not appeared. Alternatively, an individual may be unhealthy, however not suffer from a disease. For example, someone who suffers from obesity is extremely unhealthy, however they are not considered to be carrying a disease.