You can have a vegan lifestyle and be healthy, but some vegans proselytize with religious-like fervor ignoring clinical studies that show diets that include meat are healthy. They do have a point about factory farming, and instead of arguing about diet, both omnivores and vegans should focus where and how our food is produce.
True we can do with sustainable farming. But in Africa and much of the third world its not a problem man and animal live side by side and roam freely.