Since specific laboratory tests for type A infections do not exist, discussion of their epidemiology is hazardous. Most of the infections are doubtless anicteric and occult. The jaundice itself lacks clinical and biological pathognomonic features. One can imagine what confusion has appeared and remains between type A and type B infections. The epidemiological distinctions attempted under these circumstances need to be reexamined in the light of recent advances in immunology.
Krugman and Giles1 in the Willowbrook State School, New York, an institution for mentally retarded children, showed that hepatitis due to type A viruses is characterized by (a) two routes of inoculation: oral and parenteral; (b) an incubation period of 29 to 43 days, with an average duration of 35 days, regardless of the route of inoculation, if the incubation period is designated as the time interval between exposure and onset of jaundice or peak of serum glutamic oxaloacetic