The five hepatitis viruses are distinct, and the most useful way to organise them is by how they leave one person and enter the next. Two — hepatitis A and hepatitis E — are enteric: they are shed in faeces and spread by the faecal-oral route. Three — hepatitis B, C and D — are bloodborne: they spread through exposure to infected blood and body fluids. This division predicts who is at risk, how outbreaks behave and which preventive measures work.
Hepatitis A and E: enteric transmission
HAV spreads when a person ingests food or water contaminated with the faeces of an infected person, or through close physical contact with an infectious person; casual contact does not spread it. The virus survives in the environment for months. Where sanitation is poor, most children are infected before the age of 10, often without symptoms, so immunity is nearly universal and disease is uncommon. Where sanitation and hygiene are good, adults remain susceptible and outbreaks cluster in high-risk groups: people who inject drugs, men who have sex with men, people experiencing homelessness, and travellers to high-endemicity areas. The average incubation period is 28 days (range 15-50 days).
HEV follows the same enteric route, mainly through contaminated drinking water, which is why it produces large outbreaks where water and sanitation are inadequate, including refugee and displacement camps. It has four genotypes that infect humans: genotypes 1 and 2 circulate in people and spread faecally-orally, while genotypes 3 and 4 mainly infect animals and pass to humans through undercooked meat. An estimated 20 million people are infected each year, of whom about 3.3 million become symptomatic and roughly 4,400 die.
Hepatitis B, C and D: bloodborne transmission
HBV is the most infectious of the bloodborne viruses and remains infectious on surfaces for at least 7 days. In high-prevalence regions it is spread mainly from mother to child at birth and between young children; elsewhere the common routes are sexual contact, sharing needles and other injecting equipment, and needlestick or sharp injuries. It can also pass by sharing razors or toothbrushes. The perinatal route matters because the age at infection determines the chance of chronic infection: about 90% of infants infected at birth become chronic carriers, compared with about 5% of adults infected later in life.
HCV spreads mainly through direct percutaneous exposure to blood, above all by sharing needles, syringes or other injecting equipment. Transfusion of unscreened blood and reuse of inadequately sterilised medical equipment are other important routes. Perinatal and sexual transmission occur but are far less efficient, and HCV is not spread through breast milk, food, water or casual contact.
HDV is unusual: it cannot replicate without HBV, because it uses the hepatitis B surface antigen as its outer coat, so it infects only people who already carry HBV or who are exposed to both at once. It follows the same bloodborne routes as HBV, and chronic HDV infection is the most severe form of chronic viral hepatitis.
The burden of chronic infection
The global burden is dominated by chronic infection, and two viruses account for nearly all of it. An estimated 240 million people live with chronic hepatitis B, with the highest numbers in the Western Pacific and African regions, and an estimated 47 million live with chronic hepatitis C. HDV infects about 5% of people with chronic hepatitis B, roughly 12 million people worldwide. Hepatitis A and E do not cause chronic infection in immunocompetent people, apart from rare chronic hepatitis E in immunosuppressed transplant recipients. Together, chronic hepatitis B and C cause most of the deaths from viral hepatitis each year, mainly through cirrhosis and hepatocellular carcinoma.