How can normal flora become pathogenic?

Normal flora, or the collection of microorganisms that typically inhabit various sites in the human body without causing disease, can become pathogenic under certain circumstances. When these microorganisms gain access to areas of the body where they are not normally present—such as entering the bloodstream, internal organs, or sterile tissues—they can trigger infections. This shift often occurs due to breaches in physical barriers, such as cuts, surgical procedures, or underlying conditions that compromise host defenses.

This transformation from a commensal relationship to a pathogenic one emphasizes the dynamic interaction between the host and its microbiota. It illustrates that normal flora are not inherently harmful but can cause disease if the balance is disrupted or if they invade normally sterile environments.