Topley and Wilson's Microbiology and Microbial Infections, 8 Volume Set

As the number of different bacteria found in constant association with human and animal diseases grew, the question of how to establish their etiological role assumed importance. Already in the 1880s it was being recognized that, though the internal organs were normally sterile or nearly so, many surface sites and body cavities communicating with the outside had a rich bacterial flora, so the presence of an organism here was of little significance. When inflammatory lesions appeared in such places it was often difficult to decide which, if any, of the organisms present was responsible.