The primary mode of transmission of hepatitis A virus infection in a 12-year-old male with a cough is ( ).

Hepatitis A virus is the causative agent of hepatitis A. The infection is mainly transmitted by fecal-oral route. The source of infection is mostly human, and the incubation period is 15 to 50 d, with an average of 28 d. The virus often exists in the serum and feces of the patient 5 to 6 d before the patient's serum alanine aminotransferase (ALT) is elevated, and the infectiousness of the serum and feces gradually disappears after 2 to 3 weeks with the production of serum-specific antibodies.