How could diet affect the immune system and susceptibility to infectious disease?

Malnutrition, the inadequate intake of calories and/or protein, is the most common cause of immunodeficiency, which increases the risk of contracting an infectious disease. Similarly, being deficient in certain micronutrients, especially vitamin-a, vitamin-e, vitamin-d, folate, and zinc, increases susceptibility to infections.^[1] Consumption of raw, unwashed, inadequately cooked, or contaminated plant material, dairy, or meat is a common mode of acquiring infections from microbes like E. coli, Shigella, Salmonella, Listeria, Campylobacter, hepatitis A virus, norovirus, poliovirus, Giardia, Cryptosporidium, roundworms, pinworms, and tapeworms.