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Why hasn’t the U.S. closed its airports to travelers from Ebola-ravaged countries?

An Ivorian health worker screens international travelers at the Felix Houphouet Boigny International airport in Abidjan, Ivory Coast. (Legnan Koula/EPA)

Now that a man in the United States has been diagnosed with Ebola, some are asking why we haven't stopped allowing people traveling from West Africa into our airports.

Thomas Eric Duncan, the patient currently being treated in the Dallas area, boarded a flight from Liberia on Sept. 19 and arrived in Texas on Sept. 20. United Airlines said Wednesday that it was told by the CDC that Duncan had used the airline to travel from Brussels to Dulles International Airport outside Washington, D.C., before flying from Dulles to Dallas-Fort Worth.

Authorities have said repeatedly that there is no danger of anyone getting sick from flying with Duncan because he was not symptomatic at the time.

But with every connecting flight carrying passengers who have recently been in the Ebola-ravaged countries of Sierra Leone, Guinea or Liberia, the chances of another exportation increase. Scares are increasing, too: On Saturday, CDC officials were dispatched to Newark Liberty International airport after a passenger on an incoming flight from Brussels began vomiting, NBC New York reported.

Several African nations have restricted or banned air travel from Ebola-stricken countries, and airlines including Kenya Airways, British Airways, Air Cote D’Ivoire and Nigeria's Arik Air have suspended flights from the countries. Front Page Africa reported  Wednesday, though, that Kenya Airways and Air Cote D’Ivoire are expected to resume some of their so-called "Ebola flights" this month.

Others airlines have greatly reduced air travel in the region. Some of that is a natural consequence of the fact that few people, save for aid workers and government officials, are traveling in and out of the region.

But other suspensions reflect a widespread fear that a person sick with Ebola could get on a plane and potentially infect other passengers and airline crew members.

The airline industry has been trying to tamp down Ebola fears this week as more and more people ask whether there should be restrictions on who can fly into the United States.

More travel restrictions, though, aren't going to make the world safer when it comes to Ebola, according to several global public health organizations. In fact, they might make the situation worse.

Air travel restrictions ignore the way Ebola is transmitted

Ebola can only be contracted through direct contact with a sick person's bodily fluids. That means saliva, feces, urine, blood, vomit or semen. It isn't transmitted through the air, so you are more likely to catch a cold on a flight than Ebola.

“It is not an optimal measure for controlling the import of Ebola virus disease,” said chief United Nations spokesperson Stephane Dujarric. “The measure does not reflect what is known about the way in which the virus passes between people."

[RELATED: How do you get Ebola?]

The U.S. has similarly spurned travel restrictions in the face of a more infectious, though less deadly, disease like Middle East Respiratory Syndrome (MERS) for similar reasons, even when sick passengers were coming to the U.S.

The State Department has warned U.S. citizens against non-essential travel to Liberia and Sierra Leone, but there are currently no plans to alter the travel warning in the wake of diagnosis, a State Department official told The Post on Wednesday.

The screening procedures at the airport in Monrovia, the Liberian capital, have been in place for months, Deborah R. Malac, the U.S. ambassador to Liberia, told The Post in a telephone interview Thursday. The CDC's Division of Global Migration and Quarantine has worked with airport officials on screening protocols, and, Malac said, CDC officials "have confidence that everything that was supposed to have been done was done."

The restrictions are also redundant

If someone isn't exhibiting symptoms of Ebola, that person is not infectious. And one of the first symptoms of Ebola is a fever. In airports in all of the affected regions and across the world, passengers coming from flights from West Africa are being screened for elevated temperatures.

If a passenger is sick or has a fever, they won't fly. According to the CDC, "a U.S. Department of Transportation rule permits airlines to deny boarding to air travelers with serious contagious diseases that could spread during flight, including travelers with possible Ebola symptoms. This rule applies to all flights of U.S. airlines, and to direct flights (no change of planes) to or from the United States by foreign airlines."

Post by CDC.

Despite the fact that an infected passenger flew from Liberia to Dallas this month, that passenger, Duncan, was not sick -- and was therefore not contagious -- while he was traveling. And once people become symptomatic, they become very sick, very quickly.

In this case, it is unlikely that a sick person could go 10 days without seeking medical care, CDC Director Tom Frieden said on Tuesday.

"At this point, there is zero risk of transmission on the flight," Frieden said. "The illness of Ebola would not have gone on for 10 days before diagnosis. He was checked for fever before getting on the flight, and there's no reason to think that anyone on the flight that he was on would be at risk."

Travel restrictions make fighting Ebola much harder

Liberia, Guinea and Sierra Leone are already economically isolated because this epidemic has spread far wider and lasted much longer than any other Ebola outbreak in history. What those countries need most now is assistance from the world.

More flight restrictions will only make it more difficult for life-saving aid and medical professionals to reach West Africa. The restrictions already in place have proved so problematic that U.S. military forces are building an "air bridge" to get health workers and medical supplies to affected areas.

"Any discontinuation of transport will affect humanitarian aid, doctors, nurses and human resources entering the country, the transfer of biological sampling and equipment for hospitals," Daniel Menucci, a representative for the World Health Organization Travel and Transport Task Force, said in August. “All of this needs international transporting, international airlines. This will create more problems in helping the countries most affected.”

In Liberia alone, according to the WHO, more than 1,400 people have died after contracting Ebola, more than any other country. And with only one doctor for every 100,000 people in a country of 4.4 million, the country relies on health care workers from across the globe to help control the epidemic there.

[RELATED: These countries are tightening their borders over Ebola fears — against expert advice]

Controlling the outbreak in West Africa is the only way to make the rest of the world safer

Researchers who have modeled the risk of the international spread of the Ebola virus have been clear that the longer the outbreak goes on and the bigger it is, the more likely Ebola is to spread beyond West Africa to the rest of the world.

"If the epidemic continues at this growth rate and produces more and more cases, obviously the probably increases with time," said Alessandro Vespignani, a Northeastern University professor whose team has been updating its model of the chance of global exportation of Ebola cases. "That means that in October, it would be 20 percent or more, and then it will grow. The probability is increasing."

It is also the reason the U.S. has mobilized military and medical resources to aid in the fight.

"The best way to protect the U.S. is to stop the outbreak in West Africa," Beth Bell, Director of the National Center for Emerging and Zoonotic Infectious Diseases at the CDC, said in testimony before Congress last month.

Restricting air travel will make West Africa's humanitarian crisis worse

Ebola has made access to basic resources like food, clean water and basic medical care for other ailments difficult, if not impossible to obtain. Travel restrictions, according to the World Health Organization, will only make that problem worse.

“If you try to shut down air travel and sea travel, you risk affecting to a huge extent the economy, people’s livelihoods and their ability to get around without stopping the virus from traveling,” said Gregory Hartl, a WHO spokesman. “You can’t ship goods in. Sometimes these goods are basic staples people need to survive — food and fuel.”

And in the aftermath of Ebola, the long term consequences of economic and health collapse in Liberia will be the world's burden.

"If the outbreak is not stopped now, we could be looking at hundreds of thousands of people infected, with profound political and economic and security implications for all of us," President Obama said last month. "That has profound effects on all of us, even if we are not directly contracting the disease."

Mark Berman and Kevin Sieff contributed to this report.

[This post has been updated.]