Conclusion The fact that people who exercise vigorously are sick less often than average does not prove that vigorous exercise prevents illness, for whether one exercises vigorously or not depends in part on one's preexisting state of health.
The author concludes that a correlation (between vigorous exercise and being healthy) does not prove a particular causal relationship (that vigorous exercise causes one to be healthier), because the causation could be reversed (one’s existing state of health influences how much you can vigorously exercise).
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