HEV is a small non-enveloped, single-stranded RNA virus approximately 27-34 nm in diameter with a genome that is approximately 7.2 kb in length. It is the only member in the Hepeviridae family and is closely related to members of the Calicivirus family. HEV was previously known as enterically-transmitted non-A, non-B hepatitis. There are four recognised genotypes, which fall into two groups. Genotypes 1 and 2 are confined to humans and cause epidemic hepatitis associated with faecal-oral transmission while genotypes 3 and 4 are swine viruses which are common in domestic and wild pigs and infect humans as an accidental host and therefore are considered as zoonoses.