Pathogen is defined as which of the following?

• A. A disease-causing chemical.
• B. A nonpathogenic microorganism.
• C. A microorganism able to cause disease.
• D. A vaccination substitute.

Answer: (C)

A pathogen is a disease-causing agent. This means a microorganism such as a bacterium, virus, fungus, or parasite that can infect a host and lead to illness. Not all microbes cause disease—many are harmless or beneficial—so the term refers specifically to those biological agents that can produce disease. A chemical that causes disease isn’t a pathogen, and a vaccination substitute isn’t a pathogen either; vaccines use parts or inactivated forms of pathogens to stimulate immunity, not to cause disease. So the idea that a microorganism able to cause disease best captures the definition.