The seroepidemiology of hepatitis A depends on the biologic features of the agent. Hepatitis A virus (HAV) is shed in the stool, and infectivity titers are significantly higher for stool than for other body materials. As a consequence, the predominant mode of spread is through fecal-oral routes. Common-source vectors include contaminated foods, water, and bivalve mollusks. Risk factors include contact with a person with hepatitis A, attendance or employment at a day care center, recent international travel, exposure to infected food or water during an outbreak, homosexual activity, and injecting drug use. No known risk factors are identified in many cases. Almost 40% of individuals in the United States are seropositive for prior HAV infection, and rates increase with age, perhaps reflecting an aging cohort of persons infected in earlier times when the infection was more common. Not unexpectedly, this decrease in current infection rates has increased the number of susceptible persons.