What is the difference between pathogens and commensals?

• A. Pathogens are external; commensals are internal.
• B. Both always cause disease.
• C. Pathogens are harmless; commensals cause disease.
• D. Pathogens cause disease; commensals normally live in the body without causing disease.

Answer: (D)

The main idea here is how microbes affect the host, not just where they live. Pathogens have the ability to cause disease because they possess factors that help them invade, damage tissues, or evade the immune system. Commensals are the microbes that normally reside on our bodies (like skin or the gut) without causing harm and often contribute to health. They’re kept in check by our immune system and the balance of the microbiota. If the host’s defenses are weakened or the balance is disrupted, commensals can become opportunistic and cause disease, but under typical conditions they don’t.

That’s why the correct statement says pathogens cause disease while commensals normally live in the body without causing disease. The other choices are not accurate because disease isn’t simply about external versus internal location, and pathogens don’t always cause disease while commensals aren’t inherently disease-causing in a healthy state.