The 19^th century French scientist, Louis Pasteur, was instrumental in increasing our understanding of microorganisms. Since microorganisms were often found in the presence of disease, he propounded the idea that they were the cause of disease – the so-called germ theory.

Claude Bernard, a contemporary of Pasteur, maintained that the cause of disease was the “soil” – the human body –rather than the “germ” – the microorganism. Bernard said that the pathological microorganisms are opportunistic and merely taking advantage of an unhealthy condition in the body. Bernard and Pasteur were engaged in a lifelong argument on this point. It wasn’t until Pasteur was on his deathbed that he conceded. “It is the soil.”