The occurrence, transmission and termination process of infectious diseases in beef cattle herds is called the epidemic process of infectious diseases, that is, the process of developing from individuals to groups. The epidemic of any infectious disease must have three basic conditions: source of infection, transmission route and susceptible animals:

(1) Pathogenic microorganisms of infection source survive and reproduce in infected animals and are continuously expelled from the body. A sick animal is a source of infection, but the source of infection is not necessarily the infected domestic animal. Some animals have no clinical symptoms. It can also carry pathogenic microorganisms and expel the original microorganisms, and the carriers of this pathogenic microorganism are not easy to be found, and it is easier to become a source of infection.

(2) The route of transmission is the route through which the original microorganisms discharged from the source of infection re-invade another susceptible beef cattle. After feeding the feed and drinking water contaminated by tuberculosis bacilli, healthy beef cattle can infect new individuals through the digestive tract, and droplets emitted during coughing and sneezing can infect new individuals through the respiratory tract. Digestive fatigue and respiratory tract are two important infection routes.

(3) Susceptible animals, the existence of susceptible animals is the main condition for the spread of infectious diseases, for example, beef cattle are susceptible to brucellosis, but all beef cattle are injected with brucellosis vaccine, which improves the immunity of beef cattle to brucellosis. Therefore, by reducing the susceptibility of susceptible animals, the nature or prevalence of infectious diseases can be reduced or controlled.