At the turn of the 20th century, Dutch physician Christiaan Eijkman showed that disease can be caused not only bymicroorganisms but by a dietary efficiency of certain substances 1now called vitamins. In 1909 German bacteriologist Paul Ehrlich introduced the world's first bactericide, a chemical designed to killspecific kinds of bacteria with killing the patient's cells as well. 2Following the discovery of penicillin in 1928 by British bacteriologist Sir Alexander Fleming, antibiotics joined medicine's chemical armory, making the fight against bacterial infectionalmost a routine matter. Antibiotics cannot act as viruses, but 3vaccines have been used to greatly effect to prevent some of the 4deadliest viral diseases. Smallpox, once a worldwide killer, wascompletely eradicated by the late 1970s, and in the United States a 5number of polio cases dropped from 38,000 in the 1950s to less than 10 a year by the 21st century. By the middle of the 20th century scientists believed they were well on the way to treating, preventing, or eradicating manyof the most deadly infectious diseases that plagued humankind for 6centuries. And by the 1980s the medical community's confidence 7in its ability to control the infectious diseases had been shaken by 8the emergency of new types of disease-causing microorganisms. 9New cases of tuberculosis developed, caused by bacteria strainsthat were resistant to antibiotics. New, deadly infections on which 10there was no known cure also appeared, including the viruses that cause hemorrhagic fever and the human immunodeficiency virus(HTV), the cause of acquired immunodeficiency syndrome.