(1) Infectious diseases are diseases caused by pathogens and can be transmitted between people or between people and animals. Pathogens refer to bacteria, viruses, parasites and other organisms that cause infectious diseases. Hepatitis A is an acute infectious disease caused by hepatitis A virus that mainly damages the liver, so the pathogen is hepatitis A virus.

(2) Hepatitis A virus exists in the feces of patients and is mainly transmitted through the fecal-oral route. It can be known that hepatitis A is mainly caused by food transmission.

(3) The source of infection refers to the people and animals where the pathogen has grown and reproduced in the body and can spread the pathogen; the transmission route refers to the path that the pathogen passes from the source of infection to the susceptible population; the susceptible population is Refers to people who lack specific immunity to certain infectious diseases. Preventive measures for infectious diseases include: controlling the source of infection, cutting off transmission routes, and protecting susceptible groups. Therefore, to prevent hepatitis A, you must pay attention to dietary hygiene. Avoid eating uncooked food, wash raw fruits and vegetables, and develop the habit of washing hands before meals and after using the toilet to cut off the transmission route.

(4) Immunity is divided into two types, specific immunity and non-specific immunity. Non-specific immunity is present at birth and is effective against various pathogens; while specific immunity is present after birth. It is newly obtained and only works against a specific pathogen. Early vaccination of close contacts of hepatitis A patients can stimulate lymphocytes in the human body to produce antibodies against the hepatitis A virus, allowing the body to acquire immunity to hepatitis A, so this type of immunity is specific immunity.

So the answer is: (1) Hepatitis A virus

(2)A

(3)B

(4)A< /p>