Most normal microflora are either commensualistic or mutualistic, so they are generally not associated with disease.

However, if conditions are right, some can be opportunistic pathogens.  Conditions that can lead to this include immunosuppression due to injury, prior illness or chemotherapy, changes in normal microflora owing to microbial anatgonism or competition or the introduction of a member of the normal microflora into a new part of the body, such as the insertion of S. epidermidis into the urethra by nonsterile catherization.