Two large groups of viruses infect the liver in man: one group infects principally the liver, the other group infects the liver and other organs. The disease caused by the first group goes under the name of classical viral hepatitis. This disease is the most chronic infectious disease in the world. (Bader TF, Viral Hepatitis, H&H publishers, 1995, p.XI.). Until the end of 1995 the following viruses were identified:
-A (HAV) Picornavirus (Finestone SM, Kapikian AZ, Purcell RH., Science 182:1026, 1973)
-B (HBV) Hepadnavirus (Blumberg, Alter, Visnich., JAMA 195:101, 1965)
-C (HCV) Flavivirus (Choo, Kuo, et al., Science 244:359, 1989)
-D (HD.) Variate B (Raised M, et al., Gut 18:997, 1977)
-E (HE) Calicivirus (Bradley DW, et al., J. Gen. Virology 68:731, 1988)
-F (?) Picornavirus? (Candidate for HF, Med. J. Australia 164 (2): 87, 1996)
-G (HG) Flavivirus (Simon N, et al., Proc. Nat. Acad. Sci. USA 92(8):3401, 1995)
Probably many more viruses will be found to fill up the alphabet, thanks to the advancements in molecular biology.

All these viruses produce hepatocellular necrosis, i.e. hepatitis which may vary in severity from symptomless to full-blown acute and chronic forms and relative complications. Acute fulminant viral hepatitis is rare. Occult and anicteric forms constitute the majority of infections, therefore a viral hepatitis may be discovered accidentally and years later at the stage of chronicity or even at the stage of cirrhosis. While the viruses differ in their structure, antigenicity, mode of transmission, replication and infectivity, they produce very similar clinical syndromes, therefore the disease cannot be diagnosed on clinical criteria alone. Nonetheless, the diagnosis should be made as early as possible because it will permit an efficient prophylaxis on contacts.

The diagnosis of hepatitis due to any cause, biological or chemical is made by measuring compounds released in the blood by the hepatocellular necrosis of which aminotransferases are the most technically convenient at this time. They are: ALT (alanine) and AST (aspartate) aminotransferases. In acute cases their serum levels may be very high in the range of 1-2000 units. In chronic cases they may be very low and only double their normal values.

The diagnosis of hepatitis due to a viral infection requires the demonstration of the infecting virus which is accomplished by immunoserology, DNA or RNA analysis and visual identification of viral particles through immunohistology and electron microscopy.

Liver biopsy is not needed in acute cases but is necessary in chronic cases in order to assess the type, extent of the damage and complications which will be cirrhosis and hepatocellular carcinoma.