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According to the 'hygiene hypothesis,' first proposed in 1989, exposure to a variety of bacteria, viruses and parasitic worms early in life helps prime a child's immune system, much like sensory experiences program his brain. Without such early instruction, the immune system may go haywire and overreact with allergies to foods, pollen and pet dander or turn on the body's own tissue, setting off autoimmune disorders.
Many of these microorganisms evolved symbiotically with humans over millions of years -- the so-called 'old friends' theory. But where they've been eradicated, a key part of human development has been thrown off.
'The vast majority of microbes are harmless. There are only a few dozen that can cause lethal infections,' says Thomas McDade, director of the Laboratory for Human Biology Research at Northwestern University.
In 1998, about 1 in 5 children in industrialized countries suffered from allergic diseases such as asthma, allergies and rashes, according to the International Study of Asthma and Allergies in Childhood, a global research initiative. The incidence of peanut allergy in the U.S. tripled between 1997 and 2008, according to a report from Mount Sinai School of Medicine in New York.
But such diseases are still relatively rare in Africa and rural Asia, as are Type 1 diabetes and multiple sclerosis.
'The geographical distribution of allergic and autoimmune diseases is a mirror image of the geographical distribution of various infections diseases,' says a report by French researchers in a March issue of the journal Clinical & Experimental Immunology devoted to the hygiene hypothesis.
Exposure to immune-stimulating germs may also lower the risk of heart disease, according to Dr. McDade. In a study of 1,700 Filipinos followed from birth to age 21 published this year in the journal Proceedings of the Royal Society, those who grew up around chicken, pigs and dogs and had bouts of diarrhea in childhood had lower rates of C-reactive protein, an inflammation marker that has been linked to cardiovascular disease, as young adults.
There are other dangers lurking in muddy water and animal feces. Nearly 70% of the 8.8 million deaths of children under age 5 world-wide in 2008 were caused by infectious diseases, most frequently pneumonia, diarrhea and malaria, according to an analysis in the Lancet last week.
Even though rural Africa and Asia have made enormous strides in public health in the past decade, infant mortality stands at 31 per 1,000 in Namibia and 34 per 1,000 in Mongolia, compared to 7 per 1,000 in the U.S. and 3 per 1,000 in Japan.
Scientists are still working on ways to separate good germs from bad ones; in the meantime, they have a few insights: Studies have shown that children who grow up with household pets have fewer allergies and less asthma than those who don't.
Michael Bell, an infectious disease specialist and deputy director of Healthcare Quality Promotion at the Centers for Disease Control and Prevention, says that people should be vigilant about wound care since bacteria can cause problems if they get into the blood stream, and he still advocates hand-washing. 'If you're not doing it 10 times a day, you're probably not doing it enough,' he says. But he and other experts say that regular soap and water are fine in most cases. Sterilizing hands is critical mainly for health-care workers and in hospitals, where disease-causing germs are prevalent and can easily spread.
Many experts advise common sense. 'We don't want to say to children, 'OK, play by the dirty river bank and catch whatever you can,'' says Dr. Weinstock. 'But we can say there's nothing wrong with kids playing in the dirt. They don't have to live in total sanitation, and they won't die from eating something off the floor. It's probably more healthy than not.'
在湖北武漢，一個小商販的孩子躺在放置于雞籠中間的嬰兒車上
參考譯文：
根據1989年首次提出的“衛生假說”(hygiene hypothesis)理論，孩子在成長早期如果接觸多種病毒、細菌和寄生蟲，將有利于其免疫系統的發展，似乎這樣能促進大腦對其做出更好的防御準備。反之，如果缺乏這類早期接觸，免疫系統就可能出現紊亂，做出過激反應，如對食物、花粉和寵物毛屑過敏等，或者引發人體機理問題，出現自身免疫失調。
數百萬年以來，許多微生物與人類形成一種共生共棲的關系，就像我們的“老朋友”一樣；如果我們將某種微生物連根拔除，就等于錯過了人體進化歷程中的一個關鍵環節。
“絕大多數微生物都是無害的，只有幾十種微生物會引發致命感染。”美國西北大學(Northwestern University)人體生物研究實驗室(Laboratory for Human Biology Research)主任麥克戴德(Thomas McDade)說道。
“國際兒童哮喘及過敏研究”(International Study of Asthma and Allergies in Childhood)是一項全球性的調查，其數據顯示，1998年在發達國家中，只有約20%的孩子患有哮喘、過敏和疹子等過敏性疾病。紐約西奈山醫學院(Mount Sinai School of Medicine)的一個報告稱，從1997年到2008年，美國的花生過敏癥發病率增長了兩倍。
然而，上述疾病在非洲和亞洲農村地區依然相對罕見，1型糖尿病和多發性硬化也是如此。
“過敏癥和自身免疫性疾病的地域分布與各類感染性疾病的地域分布恰恰相反。”2010年一位法國研究人員在今年3月《臨床與實驗免疫學雜志》(Clinical & Experimental Immunology)上發表的一篇文章中說道。該雜志專門研究“衛生假說”理論。
麥克戴德稱，接觸那些能刺激免疫機能的微生物還可能有助于降低心臟疾病的發病率。2010年，英國《皇家學會學報》(Proceedings of the Royal Society)發表了一個報告：一項對1700名菲律賓人從出生到21歲的長期調查顯示，那些從小在有雞鴨豬狗的生活環境中長大、小時候得過幾次痢疾的人，成年后體內C反應蛋白的濃度較低。C反應蛋白是一種炎癥標志物，與心血管疾病有一定的關聯。
但臟水和動物排泄物也潛藏著其他一些危險。英國醫學雜志《柳葉刀》(Lancet)2010年五月中旬發表的一篇分析報告稱，在2008年全球880萬5歲前夭折的孩子中，有近70%死于感染性疾病，最常見的是肺炎、痢疾和瘧疾。
雖然過去10年來非洲和亞洲的農村地區在公共衛生方面有了巨大進步，但納米比亞的嬰兒死亡率仍高達千分之31，蒙古為千分之34，而美國和日本的嬰兒死亡率僅為千分之七和千分之三。
科學家仍在尋找辦法把好的微生物與壞的微生物區分開來，在此過程中，他們有了幾點深入發現。研究表明，從小和家庭寵物一起長大的孩子患過敏癥和哮喘病的幾率較低。
美國疾病控制與預防中心(Centers for Disease Control and Prevention)醫療質量改進部(Healthcare Quality Promotion)副主任及感染病專家貝爾(Michael Bell)說，人們應該警惕傷口感染，因為一旦病毒侵入血液，就會造成各種問題。他還是提倡人們勤洗手。“一天至少要洗10次手。”貝爾說。不過，他和其他一些專家表示，在大多數情況下，洗手用普通的肥皂和水就行了。手部消毒主要適用于醫療工作者以及醫院場所內，因為醫院里的致病微生物較多，而且容易傳播開來。
許多專家建議人們憑常識辦事。“我們不會對孩子們說：‘好，去骯臟的河邊玩吧，想抓什么就抓什么。’”溫斯托克(Weinstock)醫生說，“但可以這樣講，小孩子玩泥巴沒什么大不了的，他們不必生活在完全干凈的環境中，也不會因為吃掉到地上的東西就死掉。這樣說不定會使他們更健康。”
