(2) Hepatitis A virus exists in patients' feces, and mainly spreads through fecal-oral route. Therefore, hepatitis A is mainly caused by dietary transmission.
(3) The source of infection refers to people and animals whose pathogens grow and reproduce in the body and can spread pathogens; Transmission route refers to the route of pathogen transmission from the source of infection to the susceptible population; Susceptible population refers to people who lack specific immunity to an infectious disease. Preventive measures of infectious diseases include: controlling the source of infection, cutting off the route of transmission and protecting susceptible people. Therefore, to prevent hepatitis A, we need to pay attention to food hygiene, do not eat uncooked food, eat raw fruits and vegetables, and develop the habit of washing hands before and after meals.
(4) There are two kinds of immunity, specific immunity and nonspecific immunity. Nonspecific immunity is inherent in human beings and has effects on various pathogens. However, specific immunity is acquired after birth and only works on specific pathogens. For close contacts of patients with hepatitis A, early vaccination can stimulate lymphocytes in the human body to produce antibodies against hepatitis A virus, so that the body can gain immunity to hepatitis A, so this immunity belongs to specific immunity.
So the answer is: (1) Hepatitis A virus.
(2) Answer
(3)B
(4) Answer