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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.'
在湖北武汉，一个小商贩的孩子躺在放置于鸡笼中间的婴儿车上
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根据1989年首次提出的“卫生假说”(hygiene hypothesis)理论，孩子在成长早期如果接触多种病毒、细菌和寄生虫，将有利于其免疫系统的发展，似乎这样能促进大脑对其做出更好的防御准备。反之，如果缺乏这类早期接触，免疫系统就可能出现紊乱，做出过激反应，如对食物、花粉和宠物毛屑过敏等，或者引发人体机理问题，出现自身免疫失调。
数百万年以来，许多微生物与人类形成一种共生共栖的关系，就像我们的“老朋友”一样；如果我们将某种微生物连根拔除，就等于错过了人体进化历程中的一个关键环节。
“绝大多数微生物都是无害的，只有几十种微生物会引发致命感染。”美国西北大学(Northwestern University)人体生物研究实验室(Laboratory for Human Biology Research)主任麦克戴德(Thomas McDade)说道。
“国际儿童哮喘及过敏研究”(International Study of Asthma and Allergies in Childhood)是一项全球性的调查，其数据显示，1998年在发达国家中，只有约20%的孩子患有哮喘、过敏和疹子等过敏性疾病。纽约西奈山医学院(Mount Sinai School of Medicine)的一个报告称，从1997年到2008年，美国的花生过敏症发病率增长了两倍。
然而，上述疾病在非洲和亚洲农村地区依然相对罕见，1型糖尿病和多发性硬化也是如此。
“过敏症和自身免疫性疾病的地域分布与各类感染性疾病的地域分布恰恰相反。”2010年一位法国研究人员在今年3月《临床与实验免疫学杂志》(Clinical & Experimental Immunology)上发表的一篇文章中说道。该杂志专门研究“卫生假说”理论。
麦克戴德称，接触那些能刺激免疫机能的微生物还可能有助于降低心脏疾病的发病率。2010年，英国《皇家学会学报》(Proceedings of the Royal Society)发表了一个报告：一项对1700名菲律宾人从出生到21岁的长期调查显示，那些从小在有鸡鸭猪狗的生活环境中长大、小时候得过几次痢疾的人，成年后体内C反应蛋白的浓度较低。C反应蛋白是一种炎症标志物，与心血管疾病有一定的关联。
但脏水和动物排泄物也潜藏着其他一些危险。英国医学杂志《柳叶刀》(Lancet)2010年五月中旬发表的一篇分析报告称，在2008年全球880万5岁前夭折的孩子中，有近70%死于感染性疾病，最常见的是肺炎、痢疾和疟疾。
虽然过去10年来非洲和亚洲的农村地区在公共卫生方面有了巨大进步，但纳米比亚的婴儿死亡率仍高达千分之31，蒙古为千分之34，而美国和日本的婴儿死亡率仅为千分之七和千分之三。
科学家仍在寻找办法把好的微生物与坏的微生物区分开来，在此过程中，他们有了几点深入发现。研究表明，从小和家庭宠物一起长大的孩子患过敏症和哮喘病的几率较低。
美国疾病控制与预防中心(Centers for Disease Control and Prevention)医疗质量改进部(Healthcare Quality Promotion)副主任及感染病专家贝尔(Michael Bell)说，人们应该警惕伤口感染，因为一旦病毒侵入血液，就会造成各种问题。他还是提倡人们勤洗手。“一天至少要洗10次手。”贝尔说。不过，他和其他一些专家表示，在大多数情况下，洗手用普通的肥皂和水就行了。手部消毒主要适用于医疗工作者以及医院场所内，因为医院里的致病微生物较多，而且容易传播开来。
许多专家建议人们凭常识办事。“我们不会对孩子们说：‘好，去肮脏的河边玩吧，想抓什么就抓什么。’”温斯托克(Weinstock)医生说，“但可以这样讲，小孩子玩泥巴没什么大不了的，他们不必生活在完全干净的环境中，也不会因为吃掉到地上的东西就死掉。这样说不定会使他们更健康。”
