Most Clostridium infections arise from endogenous sources. That is, many of the Clostridium species that are associated with disease in humans are part of the normal intestinal microflora, which is true of C. difficile.
The organism was originally isolated in 1935 as a component of the normal intestinal flora of healthy newborns. It was dubbed "difficile" because the organism grows slowly and is difficult to culture. Early investigators also noted that the organism produced a potent toxin, but the relationship between C. difficile antibiotic-associated diarrhea (AAD) and pseudomembranous colitis (PMC) was not elucidated until the 1970's. PMC is an inflammatory disease of the colon caused by toxins of C. difficile.
Normal intestinal flora is an important factor in host response to an infectious microorganism. Resistance to intestinal infection is significantly reduced when there is a reduction in the normal flora as a result of antibiotic treatment. The most common manifestation of this decreased host resistance is the development of PMC.