2014年经济学人 埃博拉的扩展 疏通还是围堵（在线收听）

The spread of Ebola

Bridges or walls

A panicky response in the West may worsen conditions in west Africa

THE death toll from Ebola in Guinea, Liberia and Sierra Leone, the three most affected countries in west Africa, now stands at around 3,900. Among cases diagnosed outside Africa, the total is one: Thomas Duncan, a Liberian national, who died in Texas on October 8th. Yet fear of Ebola in relatively unaffected countries risks making the tragedy in Africa worse.

On October 3rd Bobby Jindal, the Republican governor of Louisiana, called for flights from “Ebola-stricken” countries to America to be suspended. Other Republican politicians have done the same. Plenty of African countries have already introduced flight bans. Some Western airlines have also altered their schedules.

Public-health experts oppose shutting countries off. Humanitarian aid and medical staff struggle to get to afflicted areas, worsening the original outbreaks. Some would-be travellers find other routes that are harder to police. The economic consequences are grave. And it is a bit more complicated than the likes of Mr Jindal imply.

Dirk Brockmann, a specialist in complex networks at Humboldt University in Berlin, has used air-transport data to study how Ebola might spread across borders. His research, which assumes that a normal flight schedule is in operation, calculates how probable it is that an infected person boarding a plane in the worst-affected countries—Guinea, Liberia and Sierra Leone, in particular—will disembark in each of dozens of other destinations.

Mr Brockmann's work offers three big messages. First, the risks to Western countries remain relatively low, even with normal flight schedules. For every 100 infected passengers embarking in Guinea, Liberia and Sierra Leone, 84 would normally disembark at another African airport. Three would arrive in Britain and France; only one in America. That infected people must get onto a plane in the first place further reduces the absolute risks. Well-prepared, well-funded health systems will be able to cope.

Second, if a country is truly intent on keeping Ebola out, it has to go further than banning flights from the worst-affected places. Global hubs are also likely to spread the disease. America's single Ebola fatality may have disembarked in Dallas but the first leg of his journey from Monrovia, Liberia's capital, was to Brussels. Airports in London and Paris play a big part in linking Sierra Leone and Guinea to the rest of the world. Calls to cut off west Africa look, in Mr Brockmann's phrase, like “19th-century thinking”. Grandstanding might be more accurate.

There is a role for better screening of passengers from affected areas; on October 8th America said it planned to start. But to screen well, it helps to know how travellers are moving around the system. That is the third lesson of the model: many passengers will find less predictable routes to their destinations if they must. Stopping flights between Conakry in Guinea and Charles de Gaulle in Paris, for example, increases the likelihood that people will fly to Dubai or to Abidjan in Ivory Coast. Others will eschew airports altogether: border crossings into Kenya are reportedly crowded with west Africans who have travelled overland in packed buses instead of taking flights. Turning your back on Ebola is not the same as stopping it.