The World Health Organization (WHO) raised the pandemic alert level on Wednesday evening to phase 5, signaling that the first influenza pandemic in more than 40 years was imminent. That announcement came on a day when the H1N1 swine flu virus continued to spread worldwide, with new cases confirmed in Austria, Germany, Britain, New Zealand and Israel, bringing the global caseload to 148.

The hardest hit countries are still Mexico and the U.S., where the virus is spreading easily from person to person — the main reason behind the WHO's decision to raise the pandemic alert from phase 4 to 5. In the U.S. there are now more than 90 confirmed cases in 10 states, with the first fatality reported on Wednesday morning — a 22-month-old Mexican boy who died in Texas. In Mexico more than 150 people have died of what is suspected to be swine flu, with more than 2,400 suspected infections — though only seven deaths and 26 cases have been confirmed as swine flu in a lab.

But the pandemic threat goes well beyond one or two countries. "An influenza pandemic should be taken seriously because of its capacity to spread rapidly," said WHO director-general Margaret Chan. "We do not have all the answers now. But we will get them."

With the H1N1 swine flu virus seemingly entrenched in both Mexico and the U.S., and continually surfacing elsewhere, a pandemic may be inevitable — flu viruses are extremely transmissible and hard to control. But that hasn't stopped health officials from trying. In the U.S. President Barack Obama said on Wednesday that affected schools "should strongly consider temporarily closing so that we can be as safe as possible," prompting the closure of schools with confirmed cases — involving some 50,000 students — in several states. At his news conference on Wednesday night, Obama also said that "every American should know that their entire government is taking the utmost precautions and preparations."

hose measures include strengthening surveillance of travelers entering the U.S., examining and testing people who might be exhibiting flu-like symptoms and alerting U.S. citizens in Mexico, where the disease still seems to be at its worst. Meanwhile, other countries have placed far stronger restrictions on travel to Mexico, in an effort to cut off the spread of the disease. Cuba and Argentina have temporarily banned flights to and from Mexico, Japan has stopped giving visas to Mexicans who arrive in the country, and France is putting forward a request to suspend all flights between the European Union and Mexico.

In hearings on Wednesday, members of Congress sharply questioned Secretary of Homeland Security Janet Napolitano on the possibility of virtually sealing off the border with Mexico. Said Republican Senator John McCain in a statement, "I continue to believe that all available options to end this crisis must remain under consideration, including closing the border if it would prevent further transmission of this deadly virus."

But Napolitano said the U.S. has no plans to close the border with Mexico, a stance that the Centers for Disease Control and Prevention (CDC) and WHO support and which Obama reiterated at his press conference. "Intensive efforts at the border are not effective means for protecting against an infectious disease," said acting CDC director Richard Besser. Still, if the swine flu continues to worsen in Mexico, it's not hard to see how Obama and other world leaders would come under increasing pressure to try to wall off Mexico — just as an infected patient might be quarantined to prevent the spread of the disease.

Health officials argue that because the H1N1 swine flu virus is already present in so many countries, and readily capable of spreading from person to person, it's far too late to try to isolate one or two countries. Although uninfected countries may be able to delay the introduction of swine flu by imposing draconian limits on international travel, they would not likely be able to stave off the virus for good — and the economic losses resulting from the travel ban may far outweigh any benefits. One 2007 study by the Brookings Institution estimated, for example, that a 95% reduction in U.S. air travel would cost the economy $100 billion a year.

"Once the virus has spread beyond its initial focus, travel restrictions just aren't effective," says Ira Longini, a biostatistician at the University of Washington. With 4,000 flights a day between the U.S. and Mexico, "it's not worth the social disruption it would cause."

That's not to say that very strict restrictions wouldn't have some effect on slowing the virus. In a 2006 study, Harvard epidemiologists John Brownstein and Kenneth Mandl examined the effect of the sharp reduction in air travel after the Sept. 11 attacks on that year's flu season. They found that the initial flight ban and general decline in air travel in the weeks after delayed the onset of the flu season but did little to reduce the overall number of infections and deaths that year.

The data matches computer models run by biostatisticians like Longini, who found that even the strictest limits on air travel would only slow the start of a flu pandemic, not stop its spread. But, again, while that strategy may benefit countries that have not yet been infected with swine flu, there's still no way to know when it would be safe to lift those restrictions. "There's no question that air travel spreads the flu," says Mandl, a physician and researcher at the informatics program at Children's Hospital Boston and an associate professor at Harvard Medical School. "But the impact of limiting flights at this point is difficult compared to the downside of the economic impact."

As far as the U.S.-Mexico border is concerned, attempting to actually close it would be futile, since countless illegal migrants cross over to the U.S. daily. Trying to stop movement may just push travelers, and the spread of the swine flu, underground. It would create a diplomatic headache as well — the Mexican government has already expressed its concern over travel restrictions.

What works better are social-distancing actions on a local level — closing schools, having employees work at home and limiting public gatherings, where the flu can spread easily. Such methods worked during the deadly 1918 Spanish flu — cities that acted quickly to close schools and theaters early in the pandemic had peak death rates 50% lower than cities that acted more slowly. Today doctors could also prophylactically administer antiviral drugs to the close contacts of any swine flu patients, a strategy that has been shown to help prevent the spread of the flu. "Until you start to see really massive clusters, that can be a really effective method," says Longini.

Ultimately, however, in a world as truly interconnected as ours, we can no more cloister a single country than we could cut off a limb. The world has become increasingly one — as the rapid spread of the swine flu virus from country to country shows. "It is really all of humanity that is under threat during a pandemic," says the WHO's Chan. Whatever happens next with the swine flu — whether it burns out or sharpens — we're in this together.

周三晚上,世界卫生组织（ WHO ）把流感警戒级别提高到5级,这就预示着40多年来第一次爆发流感的严重性.当天猪流感病毒H1N1继续全球蔓延.在奥地利，德国，英国，新西兰和以色列，也发现并证实了新的病例,从而使全球的病例达到148个。

受灾最严重的国家仍然是墨西哥和美国，在那里病毒很容易人传人-这是主要原因迫使世卫组织作出决定，将流行病警戒级别由4级提升到5级.在美国,已经有10个州确诊了超过90个病例.周三上午,发表了第一个死亡报告- 得克萨斯州一名22个月的墨西哥男婴死亡。在墨西哥,超过150人死于被怀疑的猪流感，并有超过2,400人被怀疑受到感染.-但是只有7类死亡和26例得到实验室的确认为猪流感。

但是，流感的威胁远远超过一两个国家范围。“我们应该高度重视流感,因为它的蔓延是很快的.”世卫组织总干事陈冯富珍说道。“我们现在没有解决办法。但是，我们会找到的。”

随着猪流感H1N1病毒在墨西哥和美国肆虐，并不断出现在其他地方，大规模蔓延的可能性是不可避免的-因为流感病毒易于传播又难于控制。但这无法阻止卫生官员试图控制其扩散的努力.周三,美国总统巴拉克奥巴马表示，受影响的学校“应认真考虑暂时关闭，以确保我们的安全，”由于病例的确诊迫使学校考虑停课-这么做要涉及几个州的大约50,000名学生。在周三晚上的新闻发布会上，奥巴马还表示，“每一位美国人应该知道，他们的政府正在采取全面的预防措施和准备工作。”

有关措施包括加强对进入美国游客的监测，检查和测试可能带有类似流感症状的人，提醒仍在墨西哥的美国公民，警告他们那里的形势似乎仍然很糟糕。与此同时，其他国家都已经严格限制国民去那里旅游,意图切断疾病的传播。古巴和阿根廷已暂时禁止飞机往返墨西哥，日本已经停止向墨西哥人发放签证.法国提出了请求，暂停所有往返欧洲联盟和墨西哥的航班。

在周三的听证会上，国会议员尖锐质疑国土安全部秘书珍妮纳波利塔诺关于封锁同墨西哥接壤边界的可能性。共和党参议员麦凯恩在一份声明中说： “我仍然认为，结束这场危机的所有适用的选项都应该予以考虑，包括关闭边界，这么做如果能防止这种致命的病毒进一步传播的话。”

但是纳波利塔诺说，美国没有计划关闭同墨西哥接壤的边界，这个立场，得到疾病控防中心（ CDC ）和世界卫生组织的支持.也是奥巴马在他的新闻发布会上反复强调的立场。“封锁边界不是防止传染病的有效的手段,”疾病控防中心执行主任理查德贝瑟说道。然而，如果猪流感在墨西哥继续恶化的话，就不难断定奥巴马和其他世界领导人将如何面临越来越大的压力，采取隔离措施-就像被感染的病人可能会被隔离，以防止该疾病的蔓延。

卫生官员认为，因为猪流感H1N1病毒已在许多国家存在了，并随时能够人传人，所以孤立一个,两个国家措施似乎难于奏效。虽然未被感染的国家采取严厉的限制国际旅行的措施,暂缓了受猪流感的影响，但是,他们不太可能能够阻止病毒的扩散-还有因为旅行禁令也给国家造成了经济损失,所以,这么做弊远大于利.根据布鲁金斯研究所发表的一份2007年的研究报告估计,航空旅行减少95%将给美国的经济带来年损失1,000个亿。

华盛顿大学的生物统计学家艾拉朗季尼说:”病毒一旦蔓延并肆虐的时候,任何旅行限制都会失效的.””与美墨之间每天4000个航班相比较,让病毒扰乱正常的社会生活秩序是很不值得的.”

但,这并不是说，采取非常严格的限制不会产生影响减缓病毒蔓延的。在2006年的一项研究中，哈佛大学流行病学家约翰布朗斯坦和肯纳斯曼德尔审查了在9.11之后的流感季节里由于急剧减少空中旅行所造成的影响。他们发现，最初的飞行禁令和紧随其后两周的流感季节造成航空旅行的普遍下降并没有减低整体的感染人数和死亡人数.

由生物统计学家如朗季尼通过计算机模拟的情况与统计的数据相吻合.他发现即使最严格的限制航空旅行只会迟缓禽流感疫情的扩散，但不会阻止其蔓延。但是，再说一次，虽然此战略只对尚未受感染的国家有利，但是还是没有办法知道什么时候可以安全地解除这些限制。“毫无疑问，空中旅行传播禽流感，”说这番话的是曼德尔,他是波士顿儿童医院的一位医生兼信息程序研究员以及在哈佛医学院作副教授 “但是,由于经济下滑给生活带来的影响已经很大了,如果再限制航班,势必会更加困难.”

就美墨边境的问题来说，试图关闭它实际上也是徒劳的.因为每天都有无数非法移民越过边界进入美国。任何试图阻止偷渡的做法只会逼迫他们转入地下,同时猪流感的传播也进入地下了.这将是一个头痛的外交问题.因此,墨西哥政府已对旅游限制表示关切了.

比较有效的办法就是展开地方群体间隔行动-关闭学校，要求员工在家工作，并限制公众集会,因为公共场所里禽流感容易传播。这种方法在1918年西班牙爆发流感期间很有效果致命-其城市迅速采取行动，在流感高峰期到来之前就关闭学校和电影院,死亡率低于行动比较缓慢的城市的一半。今天，医生还可以对有过密切接触猪流感病人的人实施预防性的抗病毒药物的治疗.这种策略已被证明有助于防止禽流感扩散. “只有你开始真正地看到大多数患者得到康复了，才可以说这种方法真正有效. ” 朗季尼说道。

然而，归根结底，在这个息息相关的世界上，我们只能彼此依赖而存在.这个世界已经变得越来越像一个整体了-正如流感病毒在国与国之间迅速蔓延一样,“这确实是对全人类一个巨大威胁,”世卫组织的陈说道。无论将来会发生什么-我们会在一起。