Which of the following best defines a pathogen?

• A. An organism that can cause infection
• B. A molecule that stores energy
• C. A type of immune cell
• D. A nutrient

Answer: (A)

A pathogen is an organism that can cause infection or disease. It’s the agent that, when it enters the body and multiplies, can lead to illness. This includes organisms like bacteria, viruses, fungi, and parasites. The idea is that a pathogen has the potential to cause harm, which is why this is the best definition.

The other choices describe things that are not disease-causing agents: a molecule that stores energy refers to chemical compounds like glucose or ATP used by cells; an immune cell is part of the body’s defense system, not an invader; a nutrient is something the body uses for energy and growth, not a disease-causing agent. It’s also helpful to remember that many organisms are harmless or even beneficial, and only a subset meet the definition of a pathogen. This distinction matters in infection control and when considering precautions and treatments in care settings.