Food forms the first of the humans basic survival needs. This has been recognized from the very beginning of the human development. As in most aspects, it brings greatest protection for human survival if taken properly. But, it may be a greatest source of harm if taken improperly. This article discusses the food habit followed in most of the human civilization and their benefits and demerits.
The food forms the source of energy for all living beings resident in earth. The energy from food transforms itself through a long chain we called food chain where one level (chain) feeds on lower level immediately below it but gets feed by upper level. This has maintained a stable biological structure of earth. It has been the basis of evolution and adaptation in beings.
The energy is not the only thing that is passed with food in food chain. There is yet another thing but harmful that passes through the chain. They are microbes or parasites. There is a certain amount (not exact) of microbes or parasites affecting organisms in each level of food chain. Suppose that the first level may be associated with x microbes while second level with y microbes and thus. When second level feeds on first level (lower level), the x microbes from first level are likely to enter the body of second level organism. Thus, the amount of microbes or parasite in second level organism is now x + y which is absolutely more of equal to y (original amount of microbes in second level). This amount increases as we go up higher in food chain. Thus, The total microbes (just possibility) entering an organism in any level of food chain is equal to sum of its own native microbe and summation of all microbes from its lower layer. It includes microbes from those lower levels on which the organism may not feed but enters through its food as the food may had that level as food. The relation is sort of recursive series. It can be written by 
where xi is actual amount of native parasites affecting organism at level i and Xi is the total amount of parasites that actually enters and thus has chances of affecting organism at level i. The scavengers (vultures and some uncivilized humans) have added risks from bacteria contaminating foods they eat. Thus, to conclude, the probability of organism being getting contracted by diseases is directly proportional to its level in food chain.
The hypothesis thus made above is not entirely true. When a microbe enters from lower level to upper level of food chain, it finds a great change in environment which makes it difficult to survive. Also, the antibody forces within the host organism also eliminate most of the microbes entering through food. But, still there is always an exception to it. Some microbes that entered the upper level through food may adapt the changed environment through mutation over years (as described by Darwin theory) to finally become native microbes for upper level organism. The best example to it is the incurable disease AIDS that is caused by HIV which have been likely to be originated from SIV that affected wild Apes from Africa on which the people feed (Hunter theory). Most of SIV that entered humans initially couldn't adapt human body and killed. But, over time due to less immunity in some humans, SIV did survived and finally mutated itself into HIV that finally became native human killer rather that for Apes.
Human is a very special being in food chain. His flexible feeding habit allows
him to be in any level of food chain except first as well as in multiple levels at same time. So, humans can feed entirely on vegetation (strict vegetarian) or entirely in meat of particular animal (like traditional nomads in arctic region who fed on reindeer). Still, modern man feeds on organisms from wide variety of levels at same time. Unlike food chain rule, he is not eaten by any other beings i.e. energy from humans is not passed to other organism and thus no chance of human disease to spread to animals. Thus, organisms from various levels of food chain (vegetation, mammals, Aves) are alike to human. So, s/he can feed on any one as he/she likes.
The disease in humans is peculiar. There is a long list of disease originating from food above first level (meat). In recent years, there has been outbreak of serious disease from animals. They include AIDS and more recent SARS. Both originate themselves from animals and are incurable. Unlike AIDS, SARS is more dangerous killer as it is airborne and quick. Also, bird flu is frequently reported to have been affected to humans thus causing death. There are other dangerous disease from animals like rabies, etc that have been long affected humans. Most of these disease have been reported to cause due to human contact with animals affected by it through food (AIDS through Ape eating), or rearing (SARS or Bird Flu). Here are still possibility of yet another outcome of SARS in near future as the growth of livestock for feeding continues throughout the world. This concludes that there is some truth of the proportionality rule of disease in food chain in humans too.
Prevention is better than cure. This is long accepted truth in medical science. Thus, being at lower level of food chain is far better than upper level. Humans are not far from this truth. Even though we are the master of food chain, the rule of it affects us because it makes us. We have to accept it. A being feeding at lower level will have less number of microbes entering its body through food and hence less probability of contracting native or new disease. This will ensure more healthy life. Health is wealth. In this world of global competition, where being at top means most, healthier ones are likely to enjoy that place. They will be more successful and thus better financially, physically and mentally. This causes them less frustration and increase probability of such persons making both their life and world better place to live in.
This however does not mean strict vegetarianism as described by religions. Religion forces to follow vegetarianism not out of need but as command or rule. It has to be followed in any conditions. But, there are times when following vegetarianism is not feasible (though possible). Such times, it is far better to use the human feeding flexibility for survival. Without this, life will be much harder especially in cold countries where meat is the only source of food available. The strict vegetarianism of Ramanujam, one of the greatest mathematical prodigies of twentieth century, is one example of this. During First World War, shortage of vegetation supply cost him his life.
As conclusion, being vegetarian is the safest and better way for a healthier and successful life as it is the lowest available level of food chain for humans. The lowest being sunlight that is impossible as we lack chlorophyll. But, strict vegetarianism is also not a good practice in modern fast life. However, vegetarianism must be preferred over any form of food from above levels of food chain if it is feasible. The meat must be taken just as need and not for taste. It is sort of Buddha state – don't be in extremes but just find a stable state (nirvana) of harmony in between.
