Internal medicine is the medical specialty concerned with the diagnosis, management and nonsurgical treatment of unusual or serious diseases. In North America, specialists in internal medicine are commonly called internists. Elsewhere, especially in Commonwealth nations, such specialists are often called physicians. Because their patients are often seriously ill or require complex investigations, internists do much of their work in hospitals. Formerly, many internists were not subspecialized and would see any complex nonsurgical problem; this style of practice has become much less common.

In modern urban practice, most internists are subspecialists: that is, they generally limit their medical practice to problems of one organ system or to one particular area of medical knowledge. For example, gastroenterologists and nephrologists specialize respectively in diseases of the gut and the kidneys. Internists have a lengthy clinical and scientific training in their areas of medical interest, and have special expertise in the use of drugs or other medical therapies (as opposed to surgery). While the name "internal medicine" may suggest that internists only treat problems of "internal" organs, this is not the case. Internists are trained to treat patients as whole people, not mere organ systems.

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The training and career pathways for internists vary considerably across the world.

First, they must receive the "entry-level" education required of any medical practitioner in the relevant jurisdiction. In all developed countries, entry-level medical education programs are tertiary-level courses, undertaken at a medical school attached to a university.

Programs that require previous undergraduate education are usually four or five years in length. Hence, gaining a basic medical education may typically take eight years, depending on jurisdiction and university. Following completion of entry-level training, newly graduated medical practitioners are often required to undertake a period of supervised practice before full licensure, or registration, is granted, typically one or two years. This period maybe referred to as "internship" or "conditional registration". Then, internists require specialist training in internal medicine or one of its subspecialities. In North America, this postgraduate training is often referred to as residency training; in Commonwealth countries, such trainees are often called registrars.

Training in medical specialties typically takes from three to ten years, and sometimes more, depending on specialty and jurisdiction. A medical practitioner who completes specialist training in internal medicine (or in one of its subspecialties) is an internist, or a medical specialist in the older, narrower sense. In some jurisdictions, training in internal medicine is begun immediately following completion of entry-level training, or even before. In other jurisdictions, a medical specialist must undertake generalist (un-streamed) training for one or more years before commencing specialization. Hence, depending on jurisdiction, an internist typically takes twelve or more years after commencing basic medical training \x97 five to eight years at university to obtain a basic medical qualification, and up to another six years to become a medical specialist. Internal Medicine subspecialists may also practice general internal medicine, but a particular subspecialty i.e. cardiology or pulmonology licensure is granted after completing a fellowship. (Additional training of 2-3 years)

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In the United States, there are two organizations responsible for certification of subspecialists within the field, the American Board of Internal Medicine, and the American Osteopathic Board of Internal Medicine.

The following are the subspecialties recognized by the American Board of Internal Medicine

• Cardiology, dealing with disorders of the heart and blood vessels
• Endocrinology, dealing with disorders of the endocrine system and its specific secretions called hormones
• Gastroenterology, concerned with the field of digestive diseases
• Hematology, concerned with blood, the blood-forming organs and its disorders
• Infectious disease, concerned with disease caused by a biological agent such as by a virus, bacterium or parasite
• Medical oncology, dealing with the study and treatment of cancer
• Nephrology, dealing with the study of the function and diseases of the kidney
• Pulmonology, dealing with diseases of the lungs and the respiratory tract
• Rheumatology, devoted to the diagnosis and therapy of rheumatic diseases.
• Adolescent medicine
• Clinical cardiac electrophysiology
• Critical care medicine
• Geriatric medicine
• Interventional cardiology
• Sleep medicine
• Sports medicine
• Transplant hepatology

Internists may also specialize in allergy and immunology. The American Board of Allergy, Asthma, and Immunology is a conjoint board between internal medicine and pediatrics.

The American College of Osteopathic Internists recognizes the following subspecialties.

• Allergy/Immunology
• Cardiology
• Critical care medicine
• Endocrinology
• Gastroenterology
• Geriatric medicine
• Hematology/Oncology
• Infectious diseases
• Nuclear medicine
• Nephrology
• Pulmonology
• Rheumatology

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