So Brantly was already infected with Ebola but not yet sick — and thus not yet contagious — when, on July 20, his wife and children flew from Liberia to Texas for a wedding. The doctor was scheduled to meet them in Texas a week later. He never made that flight. He fell sick three days later. An Ebola diagnosis followed. He soon made a high-security medical evacuation to Emory University Hospital in Atlanta where he eventually recovered.

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But Brantly came exceedingly close to returning to the United States with Ebola. As a doctor, he would’ve taken precautions to prevent anyone else from getting sick. However, the resulting panic — “Ebola in Texas!” — might have been impossible to contain.

A bit of luck was involved there.

And that luck seems to be holding six months into the worst Ebola outbreak in history.

Not a single reported Ebola case has made the leap from the West African outbreak to the United States or Europe — or Asia or Australia. Only two nations, Senegal and Nigeria, have seen any Ebola cases slip out of the virus’s hot zone centered on Sierra Leone, Liberia and Guinea.

Maintaining this lucky streak will only get more difficult — and soon impossible — as the outbreak grows exponentially.

“It is not unexpected that we are lucky so far,” said professor Alessandro Vespignani at Northeastern University, who runs a model projecting Ebola’s spread. There’s about a 10 percent chance of a single Ebola case getting imported to the United States in the next week. “The problem is what will happen in October, when we will likely have a much larger probability.”

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And if the epidemic reaches anything close to 1.4 million cases, the worst-case projection for four months out, then Ebola in the United States becomes a near-certainty. That doesn’t mean an outbreak, but at least one case.

http://www.washingtonpost.com/posttv/national/health-science/cdc-ebola-cases-could-reach-14-million-by-jan/2014/09/23/ac283e52-aeb7-422d-9c4e-eef17b0d4ee1_video.html

Calculating the odds that Ebola will spread is complex. It involves a bit of guesswork and lots of big data — for example, the number of international flights passing through the hot zone and the number of seats on those jets and the chances that any one of those seats will be taken by someone with an early, undetected Ebola infection.

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The odds also reflect policy choices. Those choices can alter odds dramatically.

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This is different than the probability of rain this weekend, Vespignani said. “We can affect the probability that Ebola will spread.”

So you can close borders. Cancel international flights. Step up pre-boarding screenings for fever, the virus’s earliest sign. All of these things have been tried in varying degrees. The World Health Organization has pushed countries to maintain flights and open borders, but with screening procedures. So far, those efforts have been effective, a WHO representative said.

The best defense might seem to go further.

“But you can’t seal off the world,” Vespignani said.

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Cutting off all access means help can’t reach people in the outbreak. No doctors. No surgical gloves. No biohazard suits. So the virus would spread even faster. That would push the odds of Ebola escaping higher.

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“These things compensate,” said Vespignani.

Another Ebola model is run by Dirk Brockmann at the Robert Koch Institute in Berlin. His team found that the probability, for example, that an infected person will board a flight from Freetown, Sierra Leone, and arrive in the United Kingdom is about 7 percent. For the United States, it’s 1.5 percent. Those odds might sound high, but as Brockmann explains, they mean that 67 infected individuals would need to board a plane for one of them to reach the United States. “This is indeed small,” Brockmann said.

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The problem is that the odds are getting lower. The lucky streak can not continue if the epidemic is not brought under control.

When Brantly was infected, there were only about 1,000 Ebola cases.

And he nearly got to Texas.

Now, just two months later, there have been nearly 5,900.

That number could quadruple to 20,000 by early November, health officials said.