For Vegetarians: How do you get the nutrients to stay healthy?

I two friends who are vegetarians who seem to be sick ALL THE TIME. Our friend group thinks this is due to them getting a lack of vitamins/nutrients due to a limited diet. So, for all the vegetarians out there, how do you make sure you get what you need to stay healthy?
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For clarification. They do not eat very many vegetables, and not a ton of fruit. Their diets are mainly noodles and pop tarts (one eats cheese pizza rolls too). I'm not trying to say that all vegetarians get sick a lot or anything like that.
For Vegetarians: How do you get the nutrients to stay healthy?
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