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The terms germ and microbe have been used to refer to invisible agents of disease since the nineteenth century, when scientists introduced the germ theory of disease, the idea that infections and contagious diseases are caused by microorganisms. Microbe, a shortening and alteration of microorganism , comes from the Greek prefix ���������–, “small,” and the word bios, “life.” Scientists no longer use the terms germ and microbe very much. Today they can usually identify the specific agents of disease, such as individual species of bacteria or viruses. To refer generally to agents of disease, they use the term pathogen, from the Greek pathos, “suffering,” and the suffix –g���, “producer.” They use microorganism to refer to any unicellular organism, whether disease-causing or not.