Human viral hepatitis, a major public health problem throughout the world, is caused by several different viruses. Hepatitis A virus is a member of the enterovirus genus. Hepatitis B is as yet an unclassified DNA virus. A number of unidentified viruses cause at least two different types of non-A, non-B hepatitis. The delta agent, which has been described more recently, is a transmissible agent which is dependent for its replication on a helper function provided by hepatitis B virus. Aspects of the ecology of viral hepatitis are described.