This article deals with medicine as practiced by trained professionals from ancient times to the present. Early medical traditions include those of Babylon, China, Egypt and India. The Greeks went even further, introducing the concepts of medical diagnosis, prognosis, and advanced medical ethics. The Hippocratic Oath, still taken (although significantly changed from the original) by doctors up to today, was written in Greece in the 5th century BCE. In the medieval age, surgical practices inherited from the ancient masters were improved and then systematized in Rogerius's The Practice of Surgery. Universities began systematic training of physicians around the years 1220 in Italy. During the Renaissance, understanding of anatomy improved, and the microscope was invented. The germ theory of disease in the 19th century led to cures for many infectious diseases. Military doctors advanced the methods of trauma treatment and surgery. Public health measures were developed especially in the 19th century as the rapid growth of cities required systematic sanitary measures. Advanced research centers opened in the early 20th century, often connected with major hospitals. The mid-20th century was characterized by new biological treatments, such as antibiotics. These advancements, along with developments in chemistry, genetics, and lab technology (such as the x-ray) led to modern medicine. Medicine was heavily professionalized in the 20th century, and new careers opened to women as nurses (from the 1870s) and as physicians (especially after 1970). The 21st century is characterized by highly advanced research involving numerous fields of science.