Hepatitis A Virus (HAV) is a 27nm nonenveloped, spherical, positive stranded RNA virus, classified within the genus hepatovirus of the picornavirus family and is among the smallest and structurally simplest of the RNA animal viruses. A single large polyprotein is expressed from a large open reading frame extending through most of the genomic RNA. This polyprotein is subsequently cleaved by a viral protease (3Cpro) to form three (possibly four) capsid proteins and several nonstructural proteins. HAV genomic replication occurs exclusively in the cytoplasm of the infected hepatocyte by a mechanism involving an RNA-dependent RNA polymerase. Though he clinical picture of viral (A) hepatitis is extremely variable ranging from asymptotic infection without jaundice to a fulminating disease and death in a few days, Hepatitis A is usually a mild illness and does not lead to chronic or persistent hepatitis. HAV is transmitted by the orofecal route and is still endemic throughout much of the world, including the western world. HAV exists worldwide as a single serotype with a small degree of antigenetic variation. Present evidence suggests that immunity acquired naturally (or from an inactivated hepatitis A vaccine) will protect against all human HAV strains.