Medicine is the branch of health science and the sector of public life concerned with maintaining human health or restoring it through the treatment of disease and injury. It is both an area of knowledge - a science of body systems, their diseases and treatment - and the applied practice of that knowledge. Western medical care is shared between medical professionals (physicians) and other professionals such as physician assistants, nurses and pharmacists, sometimes known as allied health professionals. Historically, only those with a medical doctorate have been considered to practice medicine. Clinicians (licensed professionals who deal with patients) can be physicians, nurses, therapists or others. The medical profession is the social and occupational structure of the group of people formally trained and authorized to apply medical knowledge. Many countries and legal jurisdictions have legal limitations on who may practice medicine. Medicine comprises various specialized sub-branches, such as cardiology, pulmonology, neurology, or other fields such as sports medicine, research or public health.