Hepatitis A virus (HAV) possesses a single-stranded linear RNA genome while hepatitis B virus (HBV) contains a double-stranded DNA genome. Detection of anti-HAV IgM in a single serum specimen obtained in the acute or convalescent stage is the quickest and most reliable method to diagnose hepatitis A infection. This antibody is usually present at onset of symptoms and may persist 3 to 6 months. Demonstration of hepatitis B surface antigen (HBsAg) in serum is the most common method of diagnosing HBV infection. Other serologic markers helpful in characterizing infection with HBV include hepatitis B surface antibody (anti-HBs), anti-hepatitis B core (anti-HBc), anti-hepatitis B e antigen (anti-HBe), and hepatitis B e antigen (HBeAg). Several epidemiologic studies have demonstrated that immune serum globulin (ISG) can prevent clinical hepatitis A even when given up to 10 days after exposure. Similar studies have shown that ISG was able to decrease the incidence of hepatitis B infection in exposed persons. Purified, noninfectious HBsAg derived from healthy HBsAg carriers has been used as a vaccine for active immunization for HBV infection. Hepatitis C is a single-stranded RNA virus belonging to the family Flaviviridae. The viral reservoir is human. Recent retrospective "lookbacks" suggest that many people were infected with HCV before testing of the blood supply was initiated in the early 1990s. HCV is treatable with combinations of drugs. The genotype of the virus plays an important role in the determination of length of therapy. Hepatitis D virus is an incomplete or defective virus which requires HBsAg as a cofactor. Both coinfection and secondary delta infection exist with secondary infection being the most serious. Hepatitis E virus is an RNA virus. Transmission is by the fecal-oral route although maternal-fetal transmission has recently been described. Prognosis is usually favorable with rare cases of fulminant HEV reported. Norwalk and Norwalk-like VirusesRotavirusOthersAdenovirusCalicivirusAstrovirusSize (nm) diameter27-357070-9035-39 Nucleic acidRNA (single-stranded)RNA (double-stranded)DNARNARNAMinimum number of serotypes34 (3 groups, A, B, C)23-55Seasonality (temperate climate)WinterWinterAll seasons--EpidemicityEpidemicSporadic, epidemicSporadicEpidemicSporadicAge with clinical disease>=6yr6-24 mo most common<=2yr<=2yr<=7TransmissionFecal-oral, water, foodFecal-oral, water, foodFecal-oralFecal-oralFecal-oral