Hepatitis B is an infection of the liver with the hepatitis B virus (HBV). HBV can cause acute or chronic hepatitis and can be transmitted perinatally as well as through percutaneous or mucosal contact with infected body fluids. Individuals at higher risk than the general population include those who have multiple sex partners, those who have unprotected sex with an infected partner, men who have sex with men, those with a history of sexually transmitted infections (STIs), and intravenous drug users. Vertical or horizontal transmission is the predominant method of transmission in some parts of the world, particularly Southeast Asia and China. Sexual contact and intravenous drug use is responsible for most infections in North America and Western Europe.

Following exposure to hepatitis B, the incubation time for acute hepatitis B is between 1 and 4 months (average 3 months). The manifestations of HBV infection are dependent on age and immune status. Infections in neonates, infants, children (10 years or younger), and immunocompromised adults are typically asymptomatic due to the absence of an immune response (immunotolerance). However, most of these patients progress to chronic HBV infection and have an increased chance for long-term, serious complications, including chronic liver disease, cirrhosis, and hepatocellular carcinoma.

In contrast, infections in older children and adults are usually symptomatic due to a more vigorous immune response. Seventy percent of patients will experience subclinical or anicteric hepatitis, while 30% will develop icteric hepatitis. Fever, urticarial rash, arthralgias, and/or arthritis are part of a serum sickness-like syndrome that may precede constitutional symptoms (fatigue, anorexia, weight loss) and jaundice, pruritus, nausea, vomiting, right upper quadrant pain, hepatomegaly, dark-colored urine, and clay-colored stools. Fulminant liver failure occurs in 0.5%-1% of cases. About 5% of adult patients will progress to chronic hepatitis, but this rate increases to 25%-50% in children and 90% in infants. Extrahepatic manifestations of acute HBV infection include the serum sickness-like illness (fever, rash, arthralgia, and/or arthritis). Extrahepatic manifestations of chronic HBV infection include polyarteritis nodosa, glomerular disease (membranous nephropathy or membranoproliferative glomerulonephritis), and cryoglobulinemia.

The presence of hepatitis B is necessary for replication of hepatitis D virus (HDV), which is more common in Mediterranean countries and some Asian countries; it is uncommon in North America. Coinfection can result in acute HBV and HDV infection. In patients with chronic hepatitis B infection, superinfection with hepatitis D may present with more severe hepatitis.