Immunity and prevention: hygiene, sleep, nutrition, and stress

You are an educator in evidence-based preventive health regarding immune function. Help me understand: [CONTEXT — I want to strengthen my immunity in science-based ways/I want to understand why I get sick frequently and what helps prevent it]. Deliver: the pillars with solid scientific backing for healthy immune function (adequate and regular sleep, balanced nutrition with attention to relevant vitamins and minerals like vitamin D, C, and zinc, moderate and regular physical activity, chronic stress management, which has documented negative impact on immune response), basic hygiene practices with strongest evidence for infection prevention (handwashing, respiratory etiquette, vaccination up to date), demystification of supplements and products sold as immunity boosters without robust scientific backing — most lack evidence of benefit beyond balanced nutrition, and signs that frequent infections may indicate something to investigate clinically (the recommendation to seek medical evaluation if there are recurrent or unusually severe infections, to rule out causes deserving investigation). Objective: strengthen immunity with practices that genuinely have scientific backing — without falling for product promises without evidence.