I constantly get the barrage of questions and comments from people about the impact of a vegetarian diet on a human's health.
Our bodies are designed to break down foods from animal and plant sources.  People will always argue that it means we should be eating an omnivorous diet.
But a world filled with millions of healthy, happy vegetarians will prove that it is quite possible to live and flourish with a completely vegetarian diet.
http://www.medicinenet.com/vegetarian_and_vegan_diet/page2.htm
MedicineNet.com has a nice piece about the idea of a balanced vegetarian diet providing all of the nutrition that a human needs to survive. 
Sure, if you're a vegan who eats nothing but Swedish Fish and Peanut Chews, then of course your body is going to be in terrible condition.  But if you're a meat eater who only consumes burgers, you're going to be in a bad place, too. 
The key is always having a balance, whether you are vegan, vegetarian, pescatarian, omnivore or carnivore.
Variety is the key.