Worldwide, geographic areas can be characterized by high, intermediate, or low levels of endemicity. Levels of endemicity are related to hygienic and sanitary conditions. In areas of high endemicity, adults are usually immune and epidemics of HA are uncommon. Improved sanitation in many parts of the world is leaving many young adults susceptible, and the frequency of outbreaks is increasing. In industrialized countries, disease transmission is most frequent among household and sexual contacts of acute cases, and occurs sporadically in day care centers with children in diapers/nappies, among travelers to countries where the disease is endemic, among injecting drug users, and among men who have sex with men. Because most children have asymptomatic or unrecognized infections, they play an important role in HAV transmission and serve as a source of infection for others. Where environmental sanitation is poor, infection is common and occurs at an early age. In some southeastern Asian areas, over 90% of the general population has serological evidence of prior HAV infection, vs. a rate of 33% in industrialized countries.

Epidemics often evolve slowly, cover wide geographic areas, and last many months; common source epidemics may evolve rapidly. During some outbreaks, day care center employees or attenders, men with multiple male sex partners and injecting drug users may be at higher risk than the general population. In about half the cases no source of infection is identified. The disease is most common among school-age children and young adults. In recent years, community-wide outbreaks have accounted for most disease transmission, although common source outbreaks due to food contaminated by food handlers and contaminated produce continue to occur and require intensive public health efforts to control. These outbreaks are usually associated with contamination of food by an HAV-infected food handler during preparation, or with food (e.g. shellfish, raw produce) contaminated before entering the food chain. Outbreaks have been reported among susceptible persons working with nonhuman primates raised in the wild.