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Obtaining nutrition and energy from food is a multi-step process. For animals, the first step is ingestion, the act of taking in food. The large molecules found in intact food cannot pass through the cell membranes. Food needs to be broken into smaller particles so that animals can harness the nutrients and organic molecules. The first step in this process is ingestion: taking in food through the mouth. Once in the mouth, the teeth, saliva, and tongue play important roles in mastication (preparing the food into bolus). Mastication, or chewing, is an extremely important part of the digestive process, especially for fruits and vegetables, as these have indigestible cellulose coats which must be physically broken down. Also, digestive enzymes only work on the surfaces of food particles, so the smaller the particle, the more efficient the digestive process . While the food is being mechanically broken down, the enzymes in saliva begin to chemically process the food as well. The combined action of these processes modifies the food from large particles to a soft mass that can be swallowed and can travel the length of the esophagus.
Besides nutritional items, other substances may be ingested, including medications (where ingestion is termed oral administration) and substances considered inedible, such as insect shells. Ingestion is also a common route taken by pathogenic organisms and poisons entering the body.
Some pathogens transmitted via ingestion include viruses, bacteria, and parasites. Most commonly, this takes place via the fecal-oral route. An intermediate step is often involved, such as drinking water contaminated by feces or food prepared by workers who fail to practice adequate hand-washing. This is more common in regions where untreated sewage is prevalent. Diseases transmitted via the fecal-oral route include hepatitis A, polio, and cholera.