Prior to the development of laboratory tests for detecting HAV and HB V infection, patients with hepatitis were usually classified on the basis of certain clinical and epidemiological features. The incubation period of hepatitis A has been defined variously as the period between exposure to infection and the onset of clinical symptoms, the onset of biochemical evidence of liver disease, or the onset of jaundice. HAV particles are shed into the feces during the incubation period of the disease and for several days or weeks after the onset of symptoms. Subclinical or inapparent infections with HAV were first described during World War II, when abnormalities of liver function and histology were detected in some experimentally infected volunteers who were apparently well. Quantitative and qualitative changes in serum immunoglobulins were first noted in some patients with hepatitis in the 1960s. Following the addition of purified HAAg, the presence of anti-HAV IgM is detected by the addition of labeled anti-HAV IgG.