The International Classification of Diseases (ICD) is a healthcare classification system maintained by the World Health Organization. It provides a set of diagnostic codes for classifying diseases (including a wide range of signs, symptoms and external causes of injury or disease) and surgical procedures. ICD codes are used for a number of different tasks, such as reporting health conditions and carrying out medical billing. The standard coding procedure consists in assigning one or more ICD codes to a patient's hospital visit: this operation is performed by medical coders, who read and review the clinical notes written by physicians and then assign the appropriate ICD codes according to the coding guidelines.