“It is also unclear why physical inactivity might be a mismatch for the immune system apart from the generally negative effects of sedentariness on overall health and levels of stress (which, as we have seen, depresses the immune system). One possibility is that because heading off to the bush to hunt and gather potentially made our ancestors more likely to encounter pathogens, our immune systems evolved to compensate by ramping up our defenses when we are active. A related possible explanation stems from the stingy way our bodies use calories. The fatigue we experience when fighting a cold is a reminder that the immune system is often energetically costly. As a result, maybe our immune systems evolved to be less vigilant when they are less needed. For hunter-gatherers, unlike most industrial people, those times might have been when they were less physically active and thus less likely to be exposed to pathogens.”