Define the term "pathogen."

The term "pathogen" refers specifically to an organism that causes disease in its host. This category includes various types of microorganisms, such as bacteria, viruses, fungi, and parasites. Pathogens can invade the body, multiply, and disrupt normal physiological functions, leading to illness or health complications. Understanding this definition is crucial in fields such as medicine and public health, as it helps identify and respond to infectious diseases effectively.

The focus on organisms that cause disease makes it clear why this choice is the correct answer, as it directly encompasses all aspects of what a pathogen is. This definition is foundational in studying how diseases spread and are treated, emphasizing the importance of recognizing pathogens in the context of health and disease management.