疫苗是人类医学史上最伟大的医学收获之一，是医学进步的里程碑。疫苗不只挽救了数以亿计的生命，更要紧的是，它还开启了对抗疾病的新思路预防。在疫苗出现之前，其他药物和手术都是直接用于治病的，而疫苗开启了预防疾病的新纪元。那样人类是怎么样了解接种疫苗的？
Back in the late 1700s, long before people understood the reason behind immunity, farmers and doctors in rural areas of Britain noticed that dairymaids and other people who got a mild disease called cowpox seldom caught its fearsome cousin, smallpox. Was there a connection?
18世纪末，早在大家理解免疫的原理之前，英国乡村区域的农民和大夫就注意到，挤牛奶的女工和其他患过一种叫牛痘的轻微病症的人极少感染牛痘可怕的表亲天花。这之间有哪些联系吗？
Some decided there was and inserted material from the cowpox into an incision they cut on the arm of healthy people, thus somehow protecting them from smallpox. In 1798, a doctor named Edward Jenner published the results of his experiments using this procedure, earning himself fame as the Father of Smallpox Vaccination.
有的人觉得有联系，并在健康人的手臂上割开一个小口，植入含有牛痘的物质，以此保护他们免于感染天花。1798年，一位名叫爱德华詹纳的大夫公布了他用这种办法的实验结果，为自己取得了天花疫苗之父的声誉。（事实上，大家目前了解，中国医士曾通过将天花结痂塞到人的鼻子里来促发免疫力，这种做法持续了数百甚或数千年。现在，一些专家觉得，吸入天花病毒或刺破皮肤等防护手段始于10世纪或11世纪的中亚。）
Note:
incision（手术）切口。
Scientists later figured out why Jenner was right that cowpox somehow protected one from smallpox. You see, when people caught cowpox, their bodies made special cells called antibodies. They fought the disease. Thats not all. They lingered in the blood in case the disease ever returned. Because cowpox and smallpox are fairly similar, if a person was later exposed to the more serious disease, anti-bodies were ready to fight it too.
科学家后来发现了詹纳提出的牛痘以某种方法保护大家免于感染天花这一说法的正确性，其原理是：当大家感染了牛痘，身体就会产生一种叫做抗体的特殊细胞，它们与疾病作斗争，不只这样，还存续在血液中，以防疾病复发；由于牛痘和天花比较相近，假如一个人后来接触到更紧急的天花，抗体也做好了与之抗争的筹备。
That knowledge helped scientists develop vaccines. The term vaccination was coined from the Latin for cow . They contain small doses of weakened, dead or modified viruses. Injected into the blood, they trick the immune system into making antibodies. If the body ever encounters those same viruses, even at full strength, the antibodies make short work of them.
这种认识能够帮助科学家研发疫苗。vaccination（疫苗接种）一词是依据拉丁语 vacca（母牛）创造的。疫苗含有少量致弱、灭活或改良的病毒。它们被注射到血液中，诱使免疫系统产生抗体。假如人体再遇见同样的病毒，即便病毒活性十足，抗体也会飞速将它消灭。
Note:
coin 设计（新单词或短语）。
at full strength 全力以赴。此处引申为（病毒）活性十足。
make short work of 非常快完成（某事）。
The injected polio vaccine, for example, is a killed, intact virus; the oral vaccine is a live, weakened virus. Vaccines for measles, mumps, chickenpox , and rubella are live, weakened viruses.
比如，注射的脊髓灰质炎疫苗是一种灭活的完整病毒；口服疫苗是一种致弱活病毒。麻疹、时尚性腮腺炎、水痘和风疹（又称德国麻疹）的疫苗都是致弱活病毒。
Vaccination prior to exposure to the virus is ideal. But a vaccination given within three days of exposure will completely prevent or significantly modify smallpox in the majority of people and given within the first four to seven days will likely offer some protection or alter the severity of the disease.
理想的状况是在接触病毒之前接种疫苗。但对大部分人来讲，接触后三天内接种疫苗仍将完全预防感染天花或显著改变感染情况；刚开始的四到七天内接种疫苗或许会起到肯定的保护用途，或减轻发病的紧急程度。