Twenty years ago, were describe the emergence of a new type of specialist that is called a "Hospitalist". Since then, the number of hospitalists has grown from a few hundred to more  than 50.000- making this new field substantially larger than any subspecialty of internal medicine, about the same size as pediatrics, and in fact larger than any specialty except general internal medicine and family medicine. Approximately 75% of U.S hospitals, including all highly ranked academic health centers, now have hospitalists. The field´s rapid growth has both reflected and contributed to the evolution of clinical practice over the past two decades. 

Source:

The New England Journal of Medicine