Regression to the mean: if the first measurement is extreme, the second is likely closer to the average. If the second measurement is extreme, the first is likely closer to the average. In the long run stocks, athletes, academics, the time it takes to fly from New York to Chicago, your immune system and your intelligence will hover around an average.
Consider the so-called SI Jinx. Cover-warranting performances are usually extraordinary, which means the athlete will regress back to his normal level of performance. Jordan was on the cover of SI so many times because his normal level was extraordinary. A coach yells at his team after a poor performance. The team plays well the next match. It appears that the coaches’ antics caused performance to improve, but the team simply regressed back to their normal level of play. You have a shitty week, go to your therapist, and the next week is normal. You would most likely have returned to normal without a visit to the therapist.
People say “I was sick, took a pill/went to the doctor, and got better” to suggest the pill/doctor visit caused improvement. This is almost always an incorrect inference, implying a causal relationship where no such thing exists. Minus bad injuries or extreme illness, the body heals itself perfectly fine without intervention. Since your immune system will normally regress back to its default state, do yourself a favor and do nothing if you experience a non-fatal illness. (no DayQuil).
Inaction is an optimal strategy much more than you think. Yet humans are plagued by a bias to do something to correct mistakes/errors/hardship when such things are commonplace events that your body and mind are perfectly prepared to handle.
Why were bleeding, vomiting and purging practiced for some 2,350 years? Doctors who performed these heinous operations were trying achieve something the body was also trying achieve: health. When bleeding, vomiting, and purging did not kill a patient they appeared to save the patient. This is a straightforward example of confusing a correlation with a cause. Ironically, the same problem still pervades medicine today: we intervene when intervention is unnecessary, and if the body self-heals we believe (mistakenly) that the intervention (and not the body’s normal recovery processes) helped. To make matters worse, we recommend the intervention to others–doctors prescribe it and big pharma mass produces it.