Have a balanced food and keep away diseases

Depends up on the contents, all foods will act either positive or negative to the immune system.

knowing this if we carefully takes the right food, it will sustain/regain our natural ability to resist deceases.

 Of course, exercise is also required, those who are not physically hard working.

Add more vegetables , fruits in diet. Avoid non-veg, too fried items, alcohol, smoking etc.Every drop of unwanted food is a  toxin which will hammer our resistive power. Similarly, a right food boost our immune power.

Foods which have antioxidants and Adaptogen  will support very strongly our defends mechanism. Mushrooms, Garlic, Green tea, Basil(thulsi), Cinnamon, ginseng and Aswaganda  contains more Adaptogen. Avoid going behind tasty food. Give simple and necessary foods to our body.Body will be happy and in turn it will give back happiness.