In the latter 19th century, the explosive growth of bacteriology transformed both the theory and the practice of medicine. This success, of course, depended on the advances made in other disciplines, yet in the popular mind the "germ theory of disease" became the symbol of progress in medical science.
Today the word "germ" has no scientific standing. If used at all in lay speech, it refers to bacteria or other infectious agents that "cause" disease. This usage, however, has only a slender connection with the original meaning, namely, something primordial or rudimentary, out of which something mature will develop. We see this sense in the metaphorical phrase, "the germ of an idea." In the earlier 19th century, the meaning of "germ" was explicitly that of a seed out of which something will grow, and the connection of seeds and disease expanded gradually over a tortuous route.
At that time, three