Define a pathogen.

A pathogen is defined as a disease-causing microorganism. This encompasses various forms of infectious agents, such as bacteria, viruses, fungi, and parasites, that can invade the body and disrupt normal physiological processes, leading to illness. The understanding of pathogens is crucial in the fields of medicine and public health, as identifying and controlling these microorganisms is key to preventing and treating infectious diseases. The other options refer to specific examples or unrelated concepts; for example, while a virus can be a type of pathogen, it does not encompass the entire definition, and a type of medicine does not pertain to the biological entities responsible for disease. Thus, the definition that most accurately describes a pathogen is one that portrays it as a microorganism capable of causing disease.