The source of infection refers to people or animals carrying pathogens; the means of transmission is the way and path of transmission of pathogens; and the susceptible group refers to the group of people who are susceptible to being infected by pathogens. 1. Infectious agents are people and animals that are infected by pathogens and in which the pathogens can survive, reproduce, and be excreted from the body. Pathogen is usually a collective term for a class of parasites and microorganisms that can cause disease. The main sources of infection are hidden infections of infectious diseases, patients with infectious diseases, and carriers of pathogens. 2. The transmission of infectious diseases is mainly between animals and humans, between humans and animals, or between humans, the common ways are digestive transmission, such as dysentery, typhoid fever, etc.; blood transmission, such as hepatitis B, syphilis, etc.; airborne transmission, such as diphtheria, measles and so on. In addition, there is contact transmission, which refers to contact with contaminated items, such as SARS, new crowns, etc.; sexual transmission, such as gonorrhea, genital herpes, etc.; vertical transmission, also known as mother-to-child transmission, generally refers to the mother in childbirth, breastfeeding to the child, such as neonatal hepatitis B, hepatitis C, etc.. 3. Susceptible people are those who lack immunity to infectious pathogens. Vaccination can generally be used to increase resistance to an infectious disease and improve susceptibility.