During the next decade and a half, researchers at many laboratories tried in vain to isolate the infectious agents that cause the two types of hepatitis. Scientists suspected that the culprit organisms were viruses because they were small enough to pass through some of the smallest-pore filters used in experiments, but the scientists were unable to grow them in order to identify and study them. By the mid-1960s, hepatitis research had reached a discouraging deadlock. Then a remarkable advance in knowledge of the causes of hepatitis was made by someone who was not working on the disease at the time. Baruch Blumberg, a medical researcher specializing in internal medicine and biochemistry, was interested in a more basic question--why were some people prone to particular diseases?

As a medical student in the early 1950s, Blumberg had conducted research in Surinam on elephantiasis, a parasitic disease common in the tropics. His investigations showed that some of the ethnic populations in the town in which he worked were more susceptible to elephantiasis than others, even though everyone was apparently exposed to the same conditions. A few years later he began to suspect that differences in susceptibility stemmed from variations in the genetic makeup of different ethnic populations, but the tools of modern molecular biology that now allow scientists to link disease susceptibility to variations in genes had not yet been invented. At the time, researchers trying to detect genetic differences that might be tied to disease susceptibility looked for inherited differences in specific blood proteins. These differences, called polymorphisms, were in some cases assumed to be maintained over generations because they gave those who carried them a survival advantage, such as resistance to a disease.

Researchers had already discovered a number of polymorphisms in blood proteins--for example, the different blood proteins that determine type A, O, or B blood--but this field was a vast and relatively unexplored terrain that promised to unlock the secrets of disease susceptibility. In the late 1950s, Blumberg embarked on research aimed at finding new polymorphisms in blood proteins. To that end he began collecting blood samples from populations all over the world.

In the early 1960s, Blumberg was at the National Institutes of Health (NIH), where he collaborated with biochemist Anthony Allison on a way to detect novel blood proteins quickly and easily. The scientists reasoned that patients who received multiple blood transfusions had probably encountered blood proteins sufficiently different from their own to prompt their bodies to generate an immune reaction, or antibodies, against the foreign proteins, or antigens. They used a technique known as agar gel diffusion, which relies on the immune system's ability to spot minor differences in proteins and to produce an antigen-antibody interaction in response to a novel blood protein.

Agar gel diffusion involves the migration of proteins and antigen-antibody complexes through gels. This technique detects the immune system's ability to spot minor differences in proteins and novel antigen-antibody interactions. First the researchers coated a glass slide with a gel, in the center of which they placed some serum from a patient who had received many transfusions. That sample was then surrounded by gel containing sera from normal people who had not received transfusions. All the serum samples diffused slowly through the gel. If any components of the normal peoples' sera reacted with antibodies from the patient's blood sample, a telltale white line appeared, indicating the presence of a combination of antigen and antibody in a concentration large enough to be detected. This reaction had two possible implications: one, that the transfused patient's blood contained antibodies that had been exposed before to antigens in the other people's sera; and two, that material found in one person's serum might be sufficiently foreign to be an antigen to another person.