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,世界衛生組織（ WHO ）把流感警戒級別提高到5級,這就預示著40多年來第一次爆發流感的嚴重性.當天豬流感病毒H1N1繼續全球蔓延.在奧地利，德國，英國，新西蘭和以色列，也發現并證實了新的病例,從而使全球的病例達到148個。

受災最嚴重的國家仍然是墨西哥和美國，在那里病毒很容易人傳人-這是主要原因迫使世衛組織作出決定，將流行病警戒級別由4級提升到5級.在美國,已經有10個州確診了超過90個病例.周三上午,發表了第一個死亡報告- 得克薩斯州一名22個月的墨西哥男嬰死亡。在墨西哥,超過150人死于被懷疑的豬流感，并有超過2,400人被懷疑受到感染.-但是只有7類死亡和26例得到實驗室的確認為豬流感。

但是，流感的威脅遠遠超過一兩個國家范圍。“我們應該高度重視流感,因為它的蔓延是很快的.”世衛組織總干事陳馮富珍說道。“我們現在沒有解決辦法。但是，我們會找到的。”

隨著豬流感H1N1病毒在墨西哥和美國肆虐，并不斷出現在其他地方，大規模蔓延的可能性是不可避免的-因為流感病毒易于傳播又難于控制。但這無法阻止衛生官員試圖控制其擴散的努力.周三,美國總統巴拉克奧巴馬表示，受影響的學校“應認真考慮暫時關閉，以確保我們的安全，”由于病例的確診迫使學校考慮停課-這么做要涉及幾個州的大約50,000名學生。在周三晚上的新聞發布會上，奧巴馬還表示，“每一位美國人應該知道，他們的政府正在采取全面的預防措施和準備工作。”

有關措施包括加強對進入美國游客的監測，檢查和測試可能帶有類似流感癥狀的人，提醒仍在墨西哥的美國公民，警告他們那里的形勢似乎仍然很糟糕。與此同時，其他國家都已經嚴格限制國民去那里旅游,意圖切斷疾病的傳播。古巴和阿根廷已暫時禁止飛機往返墨西哥，日本已經停止向墨西哥人發放簽證.法國提出了請求，暫停所有往返歐洲聯盟和墨西哥的航班。

在周三的聽證會上，國會議員尖銳質疑國土安全部秘書珍妮納波利塔諾關于封鎖同墨西哥接壤邊界的可能性。共和黨參議員麥凱恩在一份聲明中說： “我仍然認為，結束這場危機的所有適用的選項都應該予以考慮，包括關閉邊界，這么做如果能防止這種致命的病毒進一步傳播的話。”

但是納波利塔諾說，美國沒有計劃關閉同墨西哥接壤的邊界，這個立場，得到疾病控防中心（ CDC ）和世界衛生組織的支持.也是奧巴馬在他的新聞發布會上反復強調的立場。“封鎖邊界不是防止傳染病的有效的手段,”疾病控防中心執行主任理查德貝瑟說道。然而，如果豬流感在墨西哥繼續惡化的話，就不難斷定奧巴馬和其他世界領導人將如何面臨越來越大的壓力，采取隔離措施-就像被感染的病人可能會被隔離，以防止該疾病的蔓延。

衛生官員認為，因為豬流感H1N1病毒已在許多國家存在了，并隨時能夠人傳人，所以孤立一個,兩個國家措施似乎難于奏效。雖然未被感染的國家采取嚴厲的限制國際旅行的措施,暫緩了受豬流感的影響，但是,他們不太可能能夠阻止病毒的擴散-還有因為旅行禁令也給國家造成了經濟損失,所以,這么做弊遠大于利.根據布魯金斯研究所發表的一份2007年的研究報告估計,航空旅行減少95%將給美國的經濟帶來年損失1,000個億。

華盛頓大學的生物統計學家艾拉朗季尼說:”病毒一旦蔓延并肆虐的時候,任何旅行限制都會失效的.””與美墨之間每天4000個航班相比較,讓病毒擾亂正常的社會生活秩序是很不值得的.”

但,這并不是說，采取非常嚴格的限制不會產生影響減緩病毒蔓延的。在2006年的一項研究中，哈佛大學流行病學家約翰布朗斯坦和肯納斯曼德爾審查了在9.11之后的流感季節里由于急劇減少空中旅行所造成的影響。他們發現，最初的飛行禁令和緊隨其后兩周的流感季節造成航空旅行的普遍下降并沒有減低整體的感染人數和死亡人數.

由生物統計學家如朗季尼通過計算機模擬的情況與統計的數據相吻合.他發現即使最嚴格的限制航空旅行只會遲緩禽流感疫情的擴散，但不會阻止其蔓延。但是，再說一次，雖然此戰略只對尚未受感染的國家有利，但是還是沒有辦法知道什么時候可以安全地解除這些限制。“毫無疑問，空中旅行傳播禽流感，”說這番話的是曼德爾,他是波士頓兒童醫院的一位醫生兼信息程序研究員以及在哈佛醫學院作副教授 “但是,由于經濟下滑給生活帶來的影響已經很大了,如果再限制航班,勢必會更加困難.”

就美墨邊境的問題來說，試圖關閉它實際上也是徒勞的.因為每天都有無數非法移民越過邊界進入美國。任何試圖阻止偷渡的做法只會逼迫他們轉入地下,同時豬流感的傳播也進入地下了.這將是一個頭痛的外交問題.因此,墨西哥政府已對旅游限制表示關切了.

比較有效的辦法就是展開地方群體間隔行動-關閉學校，要求員工在家工作，并限制公眾集會,因為公共場所里禽流感容易傳播。這種方法在1918年西班牙爆發流感期間很有效果致命-其城市迅速采取行動，在流感高峰期到來之前就關閉學校和電影院,死亡率低于行動比較緩慢的城市的一半。今天，醫生還可以對有過密切接觸豬流感病人的人實施預防性的抗病毒藥物的治療.這種策略已被證明有助于防止禽流感擴散. “只有你開始真正地看到大多數患者得到康復了，才可以說這種方法真正有效. ” 朗季尼說道。

然而，歸根結底，在這個息息相關的世界上，我們只能彼此依賴而存在.這個世界已經變得越來越像一個整體了-正如流感病毒在國與國之間迅速蔓延一樣,“這確實是對全人類一個巨大威脅,”世衛組織的陳說道。無論將來會發生什么-我們會在一起。