A Dentist and Dental Assistant perform surgery on a patient. The World Health Organization defines Dentistry as "the science and art of preventing, diagnosing and treating diseases, injuries and malformations of the teeth, jaws and mouth". A dentist is a healthcare professional qualified to practice dentistry after graduating with a degree of either Doctor of Dental Surgery (DDS), Doctor of Dental Medicine (DMD), Bachelor of Dentistry (BDent), Bachelor of Dental Science (BDSc), or Bachelor of Dental Surgery/Chirurgiae (BDS) or (BChD) or equivalent. In most western countries, to become a qualified dentist one must usually complete at least 4 years of postgraduate study. Generally, 2 years of clinical experience working with patients in an educational setting are required. The first dental school, the Baltimore College of Dental Surgery, opened in Baltimore, Maryland in 1840. Harvard Dental School was the first dental school to affiliate with a university in 1867 (renamed Harvard School of Dental Medicine in 1940.) By 1879 dental schools were set up in London, and in England the 1878 Dentists' Act and 1879 Dentists' Register meant that only qualified and registered practitioners could hold the title of `dentist' or `dental surgeon'. The British Dental Association, formed in 1880 with Sir John Tomes as president, played a major role in prosecuting illegally practising dentists.