Medicine has always sought to understand illness and maintain wellbeing.

What constitutes health? What is illness? For many cultures in the past, illness, especially epidemics, represented the displeasure of the gods, or God. During the Black Death in the 14th century groups of penitents would publicly scourge themselves in the belief that plague had been sent as punishment for their sins. To our secular society this may seem strange, but we could perhaps draw parallels with those who feel that patients suffering diseases related to smoking, drinking or obesity have somehow brought it upon themselves through self-indulgence and lack of moral fibre.

Philosophers and scientists have developed numerous ways to understand the human body in both its 'healthy' and 'malfunctioning' states. The longest lasting have been ideas about balancing the elements of the patient's body, lifestyle and environment. This central idea appears in medical systems around the world including Ayurvedic and Chinese medicine. In European and Islamic medicine, it forms the basis of humoral theory developed by Hippocrates. Expanded and promoted by the Roman physician Galen, this theory held sway over the medical establishment for over a millennium.

The body was thought to be made up of four substances known as humours: blood, phlegm, yellow bile and black bile. A 'well' person's humours would be in a natural balance. To maintain this balance, the patient's responsibility was to lead a temperate lifestyle - regular exercise, lots of sleep, and no excesses of rich food, alcohol, sex or excitement.

An 'ill' person was thought to have an imbalance in their humours. Given that outward signs of illness often take a liquid form this seems logical: phlegm, associated with water and winter, becomes more prevalent during the cold and flu seasons. The body seemed to be trying to rid itself of this excess, and the doctor's job was to support this process through bleeding, purging and emetics, and using foods to heat, cool, moisten or dry out the patient.

During the 19th century, new thinking about the nature of illness challenged the humoral orthodoxy. Physicians had observed that certain illnesses had matching qualities and symptoms in every patient, and particular treatments seemed effective for everyone. The cause of disease was identified as something external, alien and undiscriminating, rather than internal and specific to the individual. Doctors began to treat the disease rather than the patient. However, until the development of antibiotics in the 20th century, very few of these 'alien invaders' could be treated effectively. A scratch or other minor injury could be fatal; puerperal (childbed) fever made giving birth a hazardous endeavour; pneumonia would carry off weak and elderly patients.

Throughout the centuries, professional medicine and home remedies have run parallel courses. Financial and geographical considerations left the majority of patients unable to access formal healthcare. If a patient's family were unable to pay for a doctor, or lived beyond reach of a church or charity-based medical establishment, they were thrown back on their own resources, relying on remedies handed down through the generations, herbal treatments provided by a neighbour, or quack medicines of dubious efficacy.

From the late 18th century, the state began to assume a role in its citizens' health, with government taking responsibility for ensuring clean water and sewerage, better housing and working conditions, national inoculation schemes and health campaigns. But until the establishment of a welfare state with the National Health Service in 1948, sick pay and free healthcare remained a rarity.

Today, the inhabitants of the developed world are the healthiest people in history - or at least have the potential to be so. We have access to food and safe drinking water; we have healthcare and medicine; we live and work in safe and comfortable surroundings; we have access to a wealth of information about our own wellbeing. Meanwhile, diseases like cholera and malaria remain a serious threat to life in developing countries. For all our advances in the understanding of sickness, 'health' is still far from the universal norm.