Internal medicine is concerned with the diagnosis and treatment of internal diseases, that is, those that affect internal organs or the body as a whole. A physician who practices internal medicine is, in the United States, an internist.
It is hard to define the boundaries between internal medicine and several other specialisms. In fact, in some countries all non-surgical specialisms are grouped conveniently with "internal medicine".
In the USA, there is some overlap between internal medicine and primary care (or family medicine), which is often practiced by internists.
In the UK, the specialism is still referred to as general medicine (although the combination general (internal) medicine can be found increasingly), and its practitioners are physicians or hospital physicians as distinct from surgeons.
The field on internal medicine came into existence mainly on the European continent. Until the late 18th century, medicine had been a thoroughly unscientific profession, ignorant of physiology and uninterested in experimental findings. Most medicine that was being practiced was based on the four humors and the writings of Galen and Hippocrates. There was very little interplay between "internal medicine" and surgery, which was being practiced by non-doctors.
A number of changes occurred at the end of the Enlightenment that would change the face of medicine. Perhaps the most vital one was the invention of the stethoscope by René-Théophile-Hyacinthe Laennec (1781-1826) around 1816. Although doctors had listened to breath sounds before, by putting their ear to the chest of the patient, it became much more comfortable to do so with the stethoscope.
The "internal method" was developed almost completely in the Salpêtrière hospital in Paris, where a large number of very influential doctors practiced the art of diagnosis and prognosis (although they seemed to be much less interested in curing that patient). The method was based on a rigorous history and physical examination (this was before Rudolf Virchow's Pathology and Wilhelm Röntgen's X-rays). Amongst diseases that were originally described by Salpêtrière doctors are Multiple Sclerosis and Haemochromatosis.
The "internal method" was later perfected by Sir William Osler and is practiced to this day.
The main tools of the doctor are the medical history and the physical examination, but this holds particularly true for internal medicine. Subtle descriptions of disease (e.g. cyclic shallow and deep breathing, as in Kussmaul's respiration) or physical signs (e.g. clubbing in many internal diseases) are important tools in guiding the diagnostic process. In the medical history, the "Review of Systems" serves to pick up symptoms of disease that a patient might not normally have mentioned, and the physical examination typically follows a structured fashion.
At this stage, a doctor is generally able to generate a differential diagnosis, or a list of possible diagnoses that can explain the constellation of signs and symptoms. Occam's razor dictates that, when possible, all symptoms should be presumed to be manifestations of the same disease process, but often multiple problems are identified.
In order to "narrow down" the differential diagnosis, blood tests and medical imaging are used. They can also serve screening purposes, e.g. to identify anemia in patients with unrelated complaints. Commonly performed screening tests, especially in older patients, are an X-rays of the chest, a full blood count, basic electrolytes, renal function and blood urea nitrogen.
At this stage, the physician will often have already arrived at a diagnosis, or maximally a list of a few items. Specific tests for the presumed disease are often required, such as a biopsy for cancer, microbiological culture etc.
Medicine is mainly focused on the art of diagnosis and treatment with medication, but many subspecialties administer non-surgical treatment: