The viral agent of hepatitis A is a small 27-rim RNA virus that belongs to the picornavirus group, which also includes poliomyelitis virus and coxsackie-virus. The agent is inactivated by ultraviolet light, by heating to 100 degrees centigrade for 5 minutes, and by exposure to 1:4000 formalin solution.

Hepatitis B is a viral infection of the liver usually transmitted by inoculation of infected blood or blood products. However, the antigen has been found in most body secretions, and it is known that the disease can be spread by oral or sexual contact.

Hepatitis B virus (HBA) is highly prevalent in homosexuals and intravenous drugs abusers. Other group at high risk include patients and staff at hemodialysis centers, physicians, dentists, nurses and personnel working in clinical and pathologic laboratories. Approximately 5-10% of infected individuals become carriers, providing a substantial reservoir of infection. 40-70% of infants born to HBsAg-positive mothers will develop antigens to hepatitis B in the bloodstream. Fecal-oral transmission of virus B has also been documented. The incubation period of hepatitis B is 6 weeks to 6 months. Clinical features of hepatitis A and B are similar; however, the onset in hepatitis B tends to be more insidious.

Hepatitis B virus is pleomorphic and occurs in spherical and tubular forms of different sizes. The largest of these, the Dane particle, is thought to be the complete infectious virus. The 42-nm Dane particle is composed of a core (27-nm particle) found in the nucleus of infected liver cells, and a double-shelled surface particle found in the cytoplasm. The other particles form an excess coating of the virus and contain no nucleic acid.

There are 3 distinct antigen-antibody systems that relate to HBV infection. In addition, DNA polymerase activity can be measured as a sensitive index of viral replication and infectivity.
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