Food poisoning is a general term for a number of diseases that are primarily transmitted through the ingestion of the disease agent. This can either be a bacteria (such as E. coli) or a toxin (such as botulism). Although the stomach can deal easily with small amounts of bacteria, large amounts can overwhelm the digestive tract, multiplying rapidly within the tract. Toxins are by far more serious - they can be present in even minute amounts and still be fatal.
Treatment usually consists of determining the source of contagion. It is very rare for just one person to be affected - the condition often strikes whole families, or even persons who have eaten at the same place. Once the source is identified, treatment can be targeted to the specific ailment. In the meantime, treatment is largely to insure that the patient does not dehydrate from their nausea and diarrhea.