单项选择题

When a disease of epidemic proportions threatens the public, scientists immediately get to work, trying to locate the source of affliction and find ways to combat. Vaccination is one of the effective ways to protect the 1 population of a region or country which may be 2 grave risk. The process of vaccination allows the patient"s body to 3 immunity to the virus or disease so that, if it is encountered, one can fight it 4 naturally. To accomplish this, a small weak or dead 5 of the disease is actually injected into the patient in a controlled environment, 6 his body"s immune system can learn to fight the invader 7 . Information 8 how to penetrate the disease"s defenses is 9 to all elements of the patient"s immune system in a process that occurs naturally, in which genetic information is passed from cell to cell. This makes sure that 10 the patient later come into contact with the real problem, his body is well equipped and trained to 11 with it, having already done so before.
There are, however, dangers 12 in the process. 13 , even the weakened version of the disease contained in the vaccine proves 14 much for the body to handle, resulting in the immune system 15 , and, therefore, the patient"s death. Such is the case of the smallpox vaccine, 16 to eradicate the smallpox epidemic that nearly 17 the whole Native American population and killed massive numbers of settlers. 18 1 in 10,000 people who receive the vaccine 19 the smallpox disease from the vaccine itself and dies from it. Consequently, the process, which is truly a 20 , may indeed hide some hidden curses.

A.entire
B.total
C.complete
D.complex