(1) Pathogens refer to bacteria, viruses, parasites and other organisms that cause infectious diseases. According to epidemiological analysis, influenza A virus is the pathogen of influenza A virus. The virus has no cell structure, and consists of a shell and protein's internal genetic material. The pathogen of bacterial pneumonia is bacteria, whose basic structure is cell wall.

(2) Preventive measures for infectious diseases include: controlling the source of infection, cutting off the route of transmission and protecting susceptible people.

(3) The epidemic of infectious diseases must have three links at the same time: the source of infection (people or animals that can spread pathogens), the route of transmission (the way for pathogens to leave the source of infection and reach healthy people), and the susceptible population (people who lack immunity to an infectious disease and are prone to the disease); Li Ming was infected with influenza A by his classmate (H 1N 1). Judging from the links of infectious diseases, Li Ming belongs to the susceptible population before illness, and belongs to the source of infection after illness.

So the answer is: (1) virus; gemmule

(2) cut off the route of transmission; Control the source of infection

(3) susceptible population; source of infection