Viral hepatitis is a common infectious disease that kills about 1.5 million people every year. Although hepatitis had been recognized for centuries, doctors had no idea what caused it until the 1940s when they came to suspect that a virus carried in human blood was responsible (see Hepatitis B: A Debilitating Disease).

Intensive research ensued to isolate the infectious agents causing two types of hepatitis, hepatitis A and hepatitis B. However, the breakthrough came from an independent study in which researchers were looking for immune reactions to foreign proteins in the blood of patients who had received many transfusions (see Searching the Blood for Clues).

Researchers explored why some patients who received blood transfusions developed fevers, chills, or rashes. They examined the response of the patient’s antibodies against foreign proteins, an immune reaction. By comparing the immune reactions of patients who had received many blood transfusions with the blood from people from distant parts of the world, researchers found a protein that reacted extremely rarely with the antibodies. Because the immune reaction was so rare, it indicated that it was caused by an infectious agent rather than by genetic variation in human blood. This agent was called antigen Aa (see Breakthrough Blood Sample).

As further research continued into blood proteins, scientists found that there was a link between the antigen Aa and hepatitis. Mounting evidence convinced everyone that Aa was part of the virus that caused hepatitis B (see Surprising Finding).

Consequently, the medical community recognized that they could reduce the incidence of hepatitis due to transfusions by testing the blood for the virus. Scientists developed a technique called radioimmunoassay for the accurate detection of this factor in the blood and all blood banks now routinely conduct this test (see Revolution in Blood Screening).

Scientists then began to work on developing methods for producing protective immunity against hepatitis. They found that an effective vaccine could be made from subunits of the virus, which was a novel method of vaccination at that time (see What about those Particles?). The hepatitis B vaccine also is promising for preventing deaths from liver cancer which is closely linked to hepatitis B (see A Vaccine to Prevent Liver Cancer).

Researchers have now identified other viruses causing different forms of hepatitis and vaccines have been developed for hepatitis A and C as well (see ABCs of Hepatitis Revealed).

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