At the end of the esophagus is a thick ring of a muscle called a sphincter which allows food to enter the stomach but not from the stomach back into the esophagus. Once the food enters the stomach it is mixed with the digestive juices.
Answer: That would be Viruses
Explanation:
It should most definitely be Virus, as a Virus needs a host and will genuinely be immune to what the human body throws at it. Unless it's a cure, the Virus will survive