Passage Summary: The student argues that because being immune stops you from getting sick, anyone who doesn't get sick must be immune.

Conclusion: People who don't show symptoms after exposure to staph must be immune to it.

Reasoning: Immunity prevents symptoms, and since many people exposed to the bacteria don't get sick, they must have that immunity.

Analysis: This is a classic 'Affirming the Consequent' fallacy: If P, then Q; Q; therefore P. Just because immunity causes a lack of symptoms doesn't mean it is the only thing that can cause a lack of symptoms. To parallel this, look for an argument that takes a 'If A, then B' rule and incorrectly concludes that B happening proves A happened. The correct answer will likely follow this exact structural error in a different context.