The epidemiology of viral hepatitis: an overview.

Type A hepatitis maintains itself in human populations without either an extrahuman or human reservoir. Intestinal carriers do not appear to be epidemiologically important; viremic carriers have not been demonstrated. Person-to-person transmission by the fecal-oral route is the usual mechanism. Epidemic and endemic occurrence is usually recognizable by well-defined characteristics; a "hyperendemic" patern has been documented. In some countries, but not all, the long-term trend of type A disease has been downward in recent years. Type B hepatitis is worldwide in distribution, and capable of maintaining itself by the carrier reservoir. Transmission is by multiple mechanisms, with the percutaneous route still of major importance when adequately defined in terms of subtle exposures. Contact-associated transmission probably occurs by both the oral-oral and venereal routes. The long-term trend of type B hepatitis is unknown, but some evidence suggests an increase in the United States apart from the epidemic associated with drug abuse. The existence of additional viruses of human hepatitis is suggested by data concerning transfusion-associated disease and multiple episodes in the same individual.