This will start with some good news. In modern societies, properly balanced diets will likely include all the necessary nutrients to support a robust immune system which would be instrumental in avoiding or surviving diseases such as Covid-19.
The list of nutrients linked to the human immune system is relatively short: linoleic acid (omega 6 fatty acid), vitamin A, vitamin B6, folic acid (vitamin B9), vitamin B12, vitamin C, vitamin D, vitamin E, zinc, copper, iron and selenium.
Chronic deficiencies in one or more of these nutrients will compromise some of the functions of the human immune system. Other nutrients are also important in helping the body absorb or process these nutrients.