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9.1Battle between Humans and Infections

Relationships between humans and microorganisms started when the former appeared on the Earth. These microorganisms including bacteria encompass many species beneficial to humans such as resident bacteria, which will be described later. They therefore must have exerted a great influence on the evolution and prosperity of human beings. That said, situations in which pathogenic microorganisms threaten human lives, i.e., infectious diseases, have also existed constantly as a grave menace to mankind. Traces of tuberculosis and smallpox can already be seen in mummies dating back to 1000 B.C. or before. With the progress of civilization bringing about an increase in the population and its concentration in urban areas, epidemics of infectious diseases began posing a major problem. The plague pandemic that broke out in 1348 is said to have wiped out about one third of the total population within a few decades. More than 500 years passed before the discovery of the correlation between bacteria and diseases by Robert Koch, and it was not until 1894 that Shibasaburo Kitasato discovered Yersinia pestis. Viruses, which are the causes of infectious diseases similar to bacteria, were discovered a little later. Back in the 14th century, when people did not know that infectious diseases are caused by pathogenic organisms, many would attribute the causes of plague to the locations of celestial bodies, activities of volcanoes, etc. It was not until the beginning of the 19th century that the infectious diseases, albeit their true identities remained enigmatic, were recognized as transmissible and people started to pay attention to environmental hygiene. This led to the suppression of epidemics and incidences of infections to a certain extent. Nevertheless, direct countermeasures against the infectious diseases remained impossible until the emergence of antibiotics. The first antibiotic was penicillin, which was discovered by Alexander Fleming, a Scottish biologist, from a species of mold in 1929. Since then, it was used for a wide array of clinical applications before the appearance of bacteria on which antibiotics had no effect (drug-resistant bacteria) in the 1960s. Thereafter, to this date, efforts to develop new antibiotics and the emergence of new drug-resistant strains have continued to come about one after another without any significant breakthroughs (see Column in Section 2 of Chapter 9).

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