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78 Smallpox is a disease that killed millions of people. Previously doctors in Asia and Africa had already inoculated people with smallpox but it killed many patients. Jenner’s vaccine was safer and it stopped thousands of people dying from smallpox. • Germ Theory. In the 1860s, French chemist Louis Pasteur proved the germ theory. It states that some diseases are caused by microorganisms. In the following decades, German scientist Robert Koch developed Pasteur’s work and formulated four postulates to establish a causal relationship between a causative agent and a disease. • X-ray. In 1895, German physicist WilhelmConrad Röntgen discovered X-rays. With the use of X-rays in medicine scientists could find the causes of many diseases. X-ray machines help doctors to diagnose what is wrong with a patient so they know how to treat them. • Vitamins. The study of vitamins and their role in cellular health began in the 19th century and continued into the 1900s . In 1881 Russian scientist Nikolai Lunin concluded that foods contain small quantities of unknown substances that are essential for good health. In 1912 Polish biochemist Casimir Funk isolated a complex of micronutrients and called it “vitamine” (from “vital” and “amine”), but later the word was shortened to “vitamin”. One of the first vitamins to be discovered was vitamin A. • Insulin. In the 1920s, Canadian researcher Frederick Banting and his colleagues discovered the hormone insulin. It helps balance blood sugar levels in diabetes patients and allows them to live normal lives. Before insulin, diabetes meant a slow and certain death. • Penicillin. In 1928, Alexander Fleming discovered penicillin. It was the first antibiotic. Prior to the discovery of penicillin, death could occur from minor wounds that became infected or from diseases such as strep throat. Penicillin became available during World War II. Infections that had frequently caused death became cured after a few days. • DNA (Deoxyribonucleic acid). The discovery of DNA was made by Swiss physician Friedrich Miescher. DNA is the blue print of a person. It carries genetic information in the cells of the body. • The Human Retrovirus (HIV). In the 1980s competing scientists Robert Gallo (the USA) and Luc Montagnier (France) separately discovered