Short answers, pulled from the story.
Vegetarianism is the practice of abstaining from meat, including red meat, poultry, seafood, freshwater fish, insects, and other animal flesh. Depending on the specific variation, a vegetarian diet may also exclude eggs, dairy, or honey; veganism excludes all animal products including dairy, eggs, and honey.
The word 'vegetarian' first appeared in writing attributed to actress and abolitionist Fanny Kemble in her Journal of a Residence on a Georgian Plantation, written in 1838-1839. The term was popularized by the founding of the Vegetarian Society in Manchester in 1847. Before the word existed, English-speaking plant-eaters were called 'Pythagoreans'.
A 2012 study found vegetarians had a 29% lower ischemic heart disease mortality and 18% lower overall cancer incidence compared to nonvegetarians. Research funded by the US National Institutes of Health found the average Seventh-day Adventist in California lives 4 to 10 years longer than the average Californian, attributed in part to a low-fat vegetarian diet rich in nuts and beans.
Vitamin B12 is not generally present in plants and is naturally found only in foods of animal origin. Deficiency can lead to anemia, neurological deficits, gastrointestinal problems, platelet disorders, and increased cardiovascular disease risk. Vegans can obtain B12 from manufactured fortified foods and dietary supplements; the recommended daily intake for adults 14 and older in the United States and Canada is 2.4 micrograms.
Jain vegetarianism is a strict lacto-vegetarian diet rooted in the principle of nonviolence (ahimsa) that goes beyond avoiding meat. It also excludes root vegetables such as potatoes, onions, and garlic, as well as mushrooms, figs, eggplant, and aquatic roots, because their harvesting is believed to harm dense communities of microscopic lifeforms called nigoda. Honey is also forbidden due to the violence of hive extraction.
The 1995 Simpsons episode was one of the first times on American television that vegetarians were depicted honestly rather than as one-dimensional stereotypes, according to critics. Paul and Linda McCartney voiced characters in the episode on the condition that Lisa remain a vegetarian permanently, resulting in one of the few lasting character changes in the show's history.
A 2006 United Nations initiative concluded the livestock sector is one of the top two or three most significant contributors to the most serious environmental problems worldwide. The same report estimated animal agriculture is responsible for 18% of global greenhouse gas emissions measured in 100-year carbon dioxide equivalents.