At the turn of the 20th century, Dutch physician Christiaan Eijkman showed that disease can be caused not only by microorganisms but by a dietary efficiency of certain substances【M1】______ now called vitamins. In 1909 German bacteriologist Paul Ehrlich introduced the worlds first bactericide, a chemical designed to kill specific kinds of bacteria with killing the patients cells as well.【M2】______ Following the discovery of penicillin in 1928 by British bacteriologist Sir Alexander Fleming, antibiotics joined medicines chemical armory, making the fight against bacterial infection almost a routine matter. Antibiotics cannot act as viruses, but【M3】______ vaccines have been used to greatly effect to prevent some of the【M4】______ deadliest viral diseases. Smallpox, once a worldwide killer, was completely eradicated by the late 1970s, and in the United States a【M5】______ number 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 many of the most deadly infectious diseases that plagued humankind for【M6】______ centuries. And by the 1980s the medical communitys confidence【M7】______ in its ability to control the infectious diseases had been shaken by【M8】______ the emergency of new types of disease-causing microorganisms.【M9】______ New cases of tuberculosis developed, caused by bacteria strains that were resistant to antibiotics. New, deadly infections on which【M10】______ there 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. 【M1】