Infectious diseases have four basic characteristics, as follows: first, there is a pathogen, each infectious disease is caused by a specific pathogen. Second, there is infectiousness, which is the most important difference between infectious diseases and other infectious diseases. Respiratory diseases are mainly airborne, droplet and dustborne, such as the currently prevalent New Coronavirus and SARS, while digestive diseases, such as typhoid fever and malaria, are mainly transmitted from water, food or through flies. Diseases can also be transmitted by everyday contact, either with infectious diseases of the digestive system, such as malaria, or with infectious diseases of the respiratory system, such as diphtheria. Infectious diseases transmitted through blood are seen in hepatitis B, hepatitis C, and AIDS. Thirdly, there are epidemiological characteristics, the epidemic process of infectious diseases shows different characteristics under the influence of natural and social factors. Fourthly, there is the characteristic of post-infection immunity, after the human body is infected with a pathogen, whether it is a dominant or recessive infection, it is able to produce specific immunity against the pathogen and the toxin secreted.