胡婧

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？

Some decided there was and inserted material from the cowpox into an incision1 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.” （In truth， we now know that for hundreds and perhaps thousands of years， Chinese healers had been inducing immunity by pushing smallpox scabs into a persons nose. Some experts now believe that these protective practices—inhaling smallpox virus or pricking the skin—began in Central Asia in the 10th or 11th century.）

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， antibodies were ready to fight it too.

That knowledge helped scientists develop vaccines. The term vaccination was coined2 from the Latin for cow （vacca）. 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 strength3， the antibodies make short work of4 them.

The injected polio vaccine， for example， is a killed， intact virus; the oral vaccine is a live， weakened virus. Vaccines for measles， mumps， chickenpox （Varicella）， and rubella （German Measles） are live， weakened viruses.

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.

18世紀(jì)末，早在人們理解免疫的原理之前，英國鄉(xiāng)村地區(qū)的農(nóng)民和醫(yī)生就注意到，擠牛奶的女工和其他患過一種叫牛痘的輕微病癥的人很少感染牛痘可怕的表親——天花。這之間有什么聯(lián)系嗎？

有些人認(rèn)為有聯(lián)系，并在健康人的手臂上割開一個小口，植入含有牛痘的物質(zhì)，以此保護(hù)他們免于感染天花。1798年，一位名叫愛德華·詹納的醫(yī)生公布了他使用這種方法的實(shí)驗(yàn)結(jié)果，為自己贏得了“天花疫苗之父”的聲譽(yù)。（事實(shí)上，我們現(xiàn)在知道，中國醫(yī)士曾通過將天花結(jié)痂塞到人的鼻子里來促發(fā)免疫力，這種做法持續(xù)了數(shù)百甚或數(shù)千年。如今，一些專家認(rèn)為，吸入天花病毒或刺破皮膚等防護(hù)措施始于10世紀(jì)或11世紀(jì)的中亞。）

科學(xué)家后來發(fā)現(xiàn)了詹納提出的“牛痘以某種方式保護(hù)人們免于感染天花”這一說法的正確性，其原理是：當(dāng)人們感染了牛痘，身體就會產(chǎn)生一種叫做抗體的特殊細(xì)胞，它們與疾病作斗爭，不僅如此，還存續(xù)在血液中，以防疾病復(fù)發(fā);因?yàn)榕６缓吞旎ū容^相近，如果一個人后來接觸到更嚴(yán)重的天花，抗體也做好了與之抗?fàn)幍臏?zhǔn)備。

這種認(rèn)識有助于科學(xué)家研發(fā)疫苗。vaccination（疫苗接種）一詞是根據(jù)拉丁語vacca（母牛）創(chuàng)造的。疫苗含有少量致弱、滅活或改良的病毒。它們被注射到血液中，誘使免疫系統(tǒng)產(chǎn)生抗體。如果人體再遇到同樣的病毒，即使病毒活性十足，抗體也會迅速將其消滅。

例如，注射的脊髓灰質(zhì)炎疫苗是一種滅活的完整病毒;口服疫苗是一種致弱活病毒。麻疹、流行性腮腺炎、水痘和風(fēng)疹（又稱德國麻疹）的疫苗都是致弱活病毒。

理想的情況是在接觸病毒之前接種疫苗。但對大多數(shù)人來說，接觸后三天內(nèi)接種疫苗仍將完全預(yù)防感染天花或顯著改善感染狀況;最初的四到七天內(nèi)接種疫苗可能會起到一定的保護(hù)作用，或減輕發(fā)病的嚴(yán)重程度。