Read the following materials and answer questions 55-60. 20 12 At the press conference held by the Ministry of Health on February 8, a reporter planned to launch his real name in Guangxi.

AIDS is a viral infectious disease, and 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). HIV mainly exists in the blood, semen, vaginal secretions and milk of infected people and patients. Therefore, the main transmission routes of AIDS are: sexual transmission, intravenous drug use, mother-to-child transmission, blood and blood products transmission.

In order to be popular among people, infectious diseases must have three links: the source of infection, the route of transmission and the susceptible population. Without these links, infectious diseases would not be prevalent.

So choose: D.