Polio virus is an enterovirus which means that the route of entry of this virus is through the gastrointestinal system. It is an RNA virus. Viruses can be classified into two types. Some viruses are enveloped in a lipid layer that can be destroyed by the lipolytic agents present in the digestive system. The lipid layer cannot withstand the stomach's digestive system as it is sensitive to alcohol, acid and other enzymes in the digestive system. Examples are influenza and HIV viruses. On the other hand, there are other types of viruses, which are non enveloped, called naked viruses, like the polio virus, which cannot be destroyed by acid (pH 3), ether, chloroform, bile or other proteolytic enzymes present in the digestive tract. Therefore the polio vaccine that contains attenuated strains of live polio virus, when given orally, cannot be destroyed by the digestive acids and enzymes and survives in the intestinal tract and induces local immunity in the intestinal tract.The dead virus used in the vaccine administered through injections, like the attenuated oral polio vaccine, would not be destroyed by the stomach's digestive system when taken orally.