Passage Summary: Just because healthy people exercise doesn't mean exercise makes you healthy; it might just be that you have to be healthy to exercise in the first place.

Conclusion: The correlation between vigorous exercise and lower rates of illness does not actually prove that exercise is the cause of better health.

Reasoning: The ability to engage in vigorous exercise is itself dependent on a person's initial state of health, suggesting the relationship might work in the opposite direction.

Analysis: The argument highlights a 'reverse causation' problem: it challenges a causal claim by suggesting that the supposed 'effect' (health) might actually be a prerequisite for the 'cause' (exercise). To find a parallel, look for an argument that dismisses a causal explanation for a correlation by pointing out that the two variables might influence each other in the opposite way.