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Disease usually results from an impact of the environment on the genetic makeup of the individual. Both variables contribute to its causation. In our world, both physical and human factors shape the environment in which we live, whereas evolutionary processes largely determine the diversity of genetic makeups. One of the dimensions of the individual's immediate environment that is subject to rapid change is the diet ingested. I here speak of dietary practices dictated by social or religious beliefs, which may first make manifest certain genetically determined vulnerabilities of infants.
Since the dawn of mankind, there have probably always been people who ingested a largely vegetarian diet. Recently their number has grown in the United States. To be sure, this is a varied group since some vegetarians will also ingest milk or milk and eggs, and some will even eat fish. But at the extreme are those who avoid all products