James Mosley, then chief of the Hepatitis Unit at the Communicable Disease Center (now Centers for Disease Control and Prevention-CDC), stated in 1965 that infectious hepatitis. (now hepatitis A-HA) was the only viral disease for which there was expert consensus in favor of waterborne transmission, although he also considered the possibility of waterborne poliomyelitis (Mosley 1967) . He tabulated 50 waterborne HA outbreaks worldwide; he also stated that 14 foodborne HA outbreaks had been recorded in the USA from 1952 to 1964. In those years, diagnosis was based on clinical signs and on serum transaminase levels-there was no direct test for HA (Cliver 1966) . Another problem in investigating common-source outbreaks of HA is the long incubation period (15-50 days, median 28 days), which challenges the victims to remember what they have eaten and the epidemiologists to create a coherent record of events. Once the frequent association of HA with raw shellfish consumption was recognized, it became routine to ask about shellfish consumption when HA was diagnosed: this may have imposed some bias against identification of other potential vehicles. By the publication of the WHO Manual on Food Virology in 1983, we were able to tabulate 153 foodborne HA outbreaks recorded between 1943 and 1982, as well as 117 waterborne HA outbreaks between 1895 and 1980 (Cliver 1983) .