What strengthens the immune system?

Good health: Eat right, exercise, sleep appropriately. Many supplements claim to strengthen the immune system. The only immune systems strengthened are the ones of the people who overprice and sell you something which makes them sleep better making large sums of money for something inexpensive and unproven.

Immune System: A good diet, good sleep, low alcohol use, exercise, daily multivitamins, taking meds prescribed exactly as directed, and a good mental health state.