[单项选择]Passage 4 When a disease of epidemic proportions rips into the populace, scientists immediately get to work, trying to locate the source of the affliction and find ways to combat it. Oftentimes, success is achieved, as medical science is able to isolate the parasite, germ or cell that causes the problem and finds ways to effectively kill or contain it. In the most serious of cases, in which the entire population of a region or country may be at grave risk, it is deemed necessary to protect the entire population through vaccination, so as to safeguard lives and ensure that the disease will not spread. The process of vaccination allows the patient’s body to develop immunity to the virus or disease so that, if it is encountered, one can ward it off naturally. To accomplish this, a small weak or dead strain of the disease is actually injected into the patient in a controlled environment, so that his body’s immune system can leam to fight the invader properly. Information on h
A. The passible negative outcome of administering vaccines.
B. The practical use of a vaccine to control an epidemic disease.
C. The effectiveness of vaccines in eradicating certain disease.
D. The method by which vaccines are employed against the disease.