Hepatitis A is transmitted orally, most commonly through food. The virus is shed in the stool and if a person does not wash his or her hands well enough after using the bathroom, that person is able to pass the virus on to someone else. Common situations where the virus is easily spread are in restaurants, in day-care centers where workers have to change diapers, and in many developing countries.
Even in the United States, hepatitis A is not rare. Before the vaccine was commonly used, over twentyfive thousand cases a year were reported to the CDC. This just represented the tip of the iceberg, since so many cases are mild and never even diagnosed. More recently, less than six thousand cases a year were reported to the CDC, suggesting that the immunization program is having an effect.
Thus, hepatitis A is not rare in the United States but is not common either. It is much more common overseas in developing countries, which is why doctors recommend that most oversea travelers receive either the vaccine or immune globulin (see the following question and response).