## TL;DR

Immune system 'boosters' are largely lifestyle factors, not supplements. Evidence-based: adequate sleep, regular moderate exercise, stress management, balanced diet (zinc, vitamin C, D), not smoking, moderate alcohol. 'Boosting' the immune system is a marketing myth — you want balanced, not overactive (allergies, autoimmunity).

## Core Explanation

Vitamin D: deficiency linked to increased respiratory infections. Zinc: reduces cold duration. Garlic, echinacea: modest evidence. Moderate exercise: enhances immune surveillance; intense exhaustive exercise: temporarily suppresses. Hygiene hypothesis: excessive cleanliness in early childhood may increase allergy/autoimmune risk. Vaccines: train immune system specifically and safely.

## Further Reading

- [How the Immune System Works (Sompayrac, 6th Ed)](https://www.wiley.com/en-us/How+the+Immune+System+Works%2C+6th+Edition-p-9781119542124)