So far, other than a few patients evacuated by air ambulance from West Africa, there have been no known cases of Ebola-infected people traveling from that region to other parts of the world. But could that luck be running out?
If the deadliest outbreak in history continues at its current pace, the probability of an exportation of Ebola to the United States by the end of September is between 3 and 15 percent, according to Alessandro Vespignani, a Northeastern University professor whose team has been continuously updating its model.
That range, Vespignani said, reflects the the best- and worst-case scenarios.
"These are relatively small probabilities," Vespignani said in an interview this week. "If we have very good screening procedures, then the probability could be less. If we consider the worst-case scenario, we have basically a 15 percent probability."
Vespignani said his team's calculations jibe with what several other Ebola-modeling groups have found -- that the United States is at risk, but that chance of the deadly virus spreading here is fairly small.
Speaking at the Centers for Disease Control and Prevention on Tuesday, President Obama said U.S. health officials "agree that the chances of an Ebola outbreak here ... are extremely low. We've been taking the necessary precautions, including working with countries in West Africa to increase screening at airports so that someone with the virus doesn't get on a plane for the United States.
"In the unlikely event that someone with Ebola does reach our shores ... more labs across our country now have the capacity to quickly test for the virus. We're working with hospitals to make sure that they are prepared, and to ensure that our doctors, our nurses and our medical staff are trained, are ready, and are able to deal with a possible case safely."
At a Senate hearing the same day Obama was in Atlanta, Beth Bell, director of the CDC's National Center for Emerging and Zoonotic Infectious Diseases, emphasized that the agency does not consider Ebola to be a "significant public health threat" to the United States.
Should an infected traveler arrive in the United States, Bell said, "we are confident that our public health and health care systems can prevent an Ebola outbreak here."
America's fail-safe against an Ebola outbreak starts with foreign airports. Bell said the CDC has already worked with countries experiencing active Ebola outbreaks -- Liberia, Sierra Leone, Guinea, Nigeria and Senegal -- to step up their screening of passengers leaving their countries before they board planes to other parts of the world. Similar training has also been extended to Transportation Safety Administration agents at U.S. airports.
The CDC also has quarantine stations in major U.S. airports that are designed to isolate cases of infectious disease.
The effort by the CDC, along with the Department of Health and Human Services, to prepare hospitals and healthcare workers for a potential outbreak has been ongoing since July, a spokesman for HHS said. That includes regular conference and coordination calls and webinars with state health officials and doctors.
The U.S. government is also trying to control another, more important factor: the pace of the outbreak in Africa. President Obama's announcement this week that the U.S. military would help lead an effort to bring the outbreak under control comes weeks after it became clear that the growth of the rate of Ebola infections was escalating exponentially.
Vespignani said that if that trend continues -- or worse, if it escalates -- the probability of an exportation of Ebola to the United States can increase rapidly. So far, the virus has infected 5,335 people in West Africa and caused 2,622 deaths, the WHO said Thursday.
Earlier this week, WHO Assistant Director General Bruce Aylward said Ebola's toll has doubled in the past month. And, the WHO warned, the number of cases could start doubling every three weeks.
"If the epidemic continues at this growth rate and produces more and more cases, obviously the probably increases with time," Vespignani said of the likelihood of Ebola spreading to the United States. "That means that in October, it would be 20 percent or more, and then it will grow. The probability is increasing."
If a non-symptomatic passenger boards a U.S.-bound plane and later develops symptoms of Ebola, Vespignani and others agree that it probably wouldn't constitute an outbreak, since the person and their contacts would be quickly identified and quarantined.
The spread of Ebola in West Africa is largely driven by an inability to treat sick patients because there aren't enough doctors, health facilities or supplies, along with the difficult task of tracking down the people who have come into contact with Ebola victims.
In the United States. those issues are not a concern.
"The extent of the outbreak is very, very small," Vespignani said of his projections. "It is generally less than four cases in the U.S."