Which of the following best describes a pathogen?

• A. A non-living particle that cannot infect living organisms
• B. A microorganism that lives harmlessly in or on the body
• C. A substance produced by microbes to inhibit the growth of other microbes
• D. An organism that causes disease in its host

Answer: (D)

Verified step by step guidance

1. Understand the definition of a pathogen: A pathogen is an organism that causes disease in its host.
2. Review the options given and identify key terms: 'non-living particle', 'harmless microorganism', 'substance produced by microbes', and 'organism that causes disease'.
3. Eliminate options that do not fit the definition of a pathogen, such as non-living particles (which cannot infect) and harmless microorganisms (which do not cause disease).
4. Recognize that substances produced by microbes to inhibit other microbes are called antibiotics, not pathogens.
5. Conclude that the best description of a pathogen is 'an organism that causes disease in its host', as it directly relates to the ability to cause illness.