Which of the following terms refers to harmless bacteria in the human body?

The term that refers to harmless bacteria in the human body is non-pathogenic. These bacteria typically coexist with humans and play numerous beneficial roles, such as aiding in digestion and protecting against harmful pathogens by occupying niches in the body that pathogens might otherwise exploit. Non-pathogenic bacteria can help maintain a healthy balance and are crucial for various bodily functions.

By contrast, the other terms have different meanings related to harmful organisms or conditions. Pathogens are specific microorganisms that can cause disease, while virulent refers to the severity or harmfulness of a pathogen’s ability to cause disease. Symptomatic relates to the presence of symptoms indicating a disease or condition rather than referring to the nature of bacteria directly. Thus, non-pathogenic is the correct choice as it accurately describes bacteria that are harmless to the human body.