Immune boosters:

Sunlight – Vitamin D and sunlight make our immune system very strong. The importance of vitamin D is so high that it protects us from infections, cancer, obesity, bone problems, auto immune diseases and every possible problem. Vitamin D is highly underestimated by us. All the statements above are 100% proven by science. For eg 60% breast cancers can be simply avoided by simply spending 20 minutes everyday in sunlight by women.
Vitamin C – Neutrophils need very very high concentration of vitamin C around them to fight pathogens. Vitamin C has been found to be safe at very high levels of intake also. Humans like most other animals had the capacity to produce Vitamin C. Out of the 4 genes required we still have 3. Since we started eating fruits and vegetables for millions of years we stopped producing vitamin C. I have personally seen people getting cured of typhoid and other fevers by taking Fruits like lemons and Amla in very high dosage and eliminating immune lowering foods.
In many cases fasting has also shown to boost immunity. Especially in case if cancer patients. Fasting in them leads to destruction of cancer cells.
Nutrient rich low calorie natural whole foods.
Probiotics and fermented foods – our body has ten times more bacteria than our own cells. So a word like infection is a joke. The most easy way to keep infections away is to simply have more good bacteria to fight on our side. They will compete for food and space and eliminate bad bacteria. For this ofcourse we need to eat food which the good bacteria eat and the bad bacteria Dont. That food is fibre.
Cheers