Doctors whose main professional focus is hospital medicine are known as hospitalists. The major function of a hospitalist is to act as a case manager of such patients. The need for hospitalists has arisen due to the tremendous growth in medical knowledge and resultant number of medical specialists whose care must be coordinated. Hospital medicine, like emergency medicine, is a specialty based on the site of patient care (the hospital), as opposed to an organ (i.e., ophthalmology), a disease (i.e., oncology), or a patient's age (i.e., geriatrics). As a relatively new specialty, hospital medicine has only been offered as a certification for specialty experience since 2009. There are more than 30,000 hospitalists currently practicing in the United States.