Which of the following defines a pathogen?

A pathogen is defined as an organism capable of causing disease in its host. This definition encompasses a wide range of organisms, including bacteria, viruses, fungi, and parasites, all of which can disrupt normal physiological functions and lead to illness. The key aspect of a pathogen is its ability to invade a host and elicit a response that often results in disease.

Other options represent concepts that are not synonymous with pathogens. Beneficial bacteria contribute to health, particularly in digestion and immune function, while compounds that enhance immunity serve to bolster the body's defenses against disease but do not cause illness themselves. Additionally, certain viruses can be used in vaccines to induce immunity without causing the disease, but they are not classified as pathogens in that context. Thus, option A accurately encapsulates the essence of what a pathogen is.