Is the cambridge diet safe when you are ill with flu and laryngitis and have very weak immune system? is it ok to not eat proper food?

No: Ask your own doctor first. But i'd recommend caution with any diet plan that relies on "replacement" food from one company. It's never "ok to not eat proper food, " as it isn't sustainable without continuing to buy company food. The foods may have various nutrients, but with the flu you may not be able to stomach such processed products. Why not learn how to cook & eat healthily instead?