Viral hepatitis is an infection of the liver that affects people from all walks of life regardless of age, race, gender, or sexual orientation. There are several different viruses that cause hepatitis. They are called hepatitis A, B, C, D, and E viruses. The viruses are transmitted in different ways. Complications include chronic liver disease, liver failure, and liver cancer. Generally, the virus is transmitted through feces, blood, or bodily fluid that contains blood. Fecal contamination of water and food, eating undercooked shellfish, and using contaminated intravenous drug needles are all common routes of transmission. Toxins like alcohol and anti-HIV drugs can lead to nonviral hepatitis.

Hepatitis A is an enterovirus transmitted by the orofecal route, such as contaminated food. It causes an acute form of hepatitis and does not have a chronic stage. The patient's immune system makes antibodies against Hepatitis A that confer immunity against future infection. A vaccine is available that will prevent infection from hepatitis A. Hepatitis A virus infection can cause an acute, flu-like illness with yellowing of the skin (jaundice), nausea and vomiting, fatigue, loss of appetite, abdominal pain, or diarrhea. It lasts from three to six weeks, but can persist up to six months. Most patients recover with no serious long-term health problems. Symptoms are more severe in adults than in children, who often have no symptoms. HAV is spread when infected human feces is ingested by mouth.

Hepatitis B causes both acute and chronic hepatitis in some patients who are unable to eliminate the virus. Identified methods of trasmission include blood (blood transfusion, now rare), tattoos (both amateur and professionally done), sexually or vertically (from mother to her unborn child). However, in about half of cases the source of infection cannot be determined. Blood contact can occur by sharing syringes in intravenous drug use, shaving accessories such as razor blades (as on the hit TV show Survivor), or touching wounds on infected persons.

Needle-exchange programmes have been created in many countries as a form of prevention. In the United States, 95% of patients clear their infection and develop antibodies against Hepatitis B virus. However, 5% of patients do not clear the infection and develop chronic infection. Only these people are at risk of long term complications of Hepatitis B. Patients with chronic hepatitis B have antibodies against Hepatitis B, but these antibodies are not enough to clear the infection that establishes itself in the DNA of the affected liver cells. The continued production of virus combined with antibodies is a likely cause of immune complex disease seen in these patients.

A vaccine is available that will prevent infection from hepatitis B. Hepatitis B infections result in 500,000 to 1,200,000 deaths per year worldwide due to the complications of chronic hepatitis, cirrhosis, and hepatocellular carcinoma. Hepatitis B is endemic in a number of (mainly South-East Asian) countries, making cirrhosis and hepatocellular carcinoma big killers.

Hepatitis C (originally "non-A non-B hepatitis") is probably not transmitted sexually but only by blood contact. It leads to a chronic form of hepatitis, culminating in cirrhosis. It can remain asymptomatic for 10-20 years. No vaccine is available for hepatitis C. However, patients with hepatitis C are prone to severe hepatitis if they contract either hepatitis A or B. Therefore all hepatitis C patients should be immunized against Hepatitis A and Hepatitis B if they are not already immune.

Two other hepatitisviruses are known, hepatitis D and E. The D agent, an RNA passenger virus, cannot proliferate without the presence of hepatitis B virus, because its genome lacks certain essential genes. Hepatitis E produces a picture quite similar to hepatitis A, although it can take a fulminant course in some patients, particularly pregnant women; it is more prevalent in the Indian subcontinent. Hepatitis D is caused by the hepatitis D virus (HDV). It occurs only in people who have hepatitis B. Hepatitis E is caused by the hepatitis E virus (HEV), which can be found in the stool (bowel movements) of infected people. It is uncommon in the United States but is a risk to international travelers.