It's very lucrative as it helps to bring money into hospitals.
Hospitals get paid for every patient that comes through their doors.
In short, you generate a series of codes derived from diagnoses and treatments, put them into a database and it generates statistics for hospital, local authority, NHS, government use and money from local health authorities and the government.
It requires strict adherence to a set of rules, going through patient records for information, taking down the reasons for admission or visit and any of a number of specific co-morbidities, looking the terms up in an index and confirming the codes in the main books.
It requires a good knowledge of anatomy and physiology (commonly referred to as A&P), a good knowledge of operations and procedures and a strong stomach - the patient notes can be pretty distressing to read and pictures are graphic.
For trainees, there is a 21/22 day initial introduction style training course and there are refresher and discipline specific courses (T&O, dermatology, oncology etc).
After 3 years of hard study and working, there is a two part exam which then allows you to call yourself accredited and signals the start of a flurry of phone calls and emails from agencies crying out for qualified coders as the governing body announces the names of all those who pass in a newsletter.