A third American infected with Ebola is coming home.

Physician Rick Sacra, 51, was infected with Ebola in Liberia, according to The Associated Press. He will be treated at a bio-containment unit at The Nebraska Medical Center in Omaha, Nebraska.

Previous Americans with Ebola were treated at Emory Hospital in Atlanta with an experimental drug called ZMapp. However, that drug’s supply has been exhausted.

In West Africa, 3,500 cases of Ebola are suspected or confirmed with 1,900 deaths, according to the World Health Organization. It is the worst Ebola outbreak in history.

ZMapp, an immune system booster, may help an infected patient to ride out the storm, but several have died from Ebola even after taking it.

The U.S. Department of Health and Human Services announced Wednesday that is it providing more than $40 million to further develop ZMapp.

Meanwhile, scientists are working at an unprecedented pace on a “trojan horse” vaccine, which they hope will be the ultimate solution to the deadly virus.

“This is moving at an incredibly fast pace,” said Myron Levine, who is in Mali to lead the West African arm of the study for the University of Maryland. “That is groundbreaking – historic.”

Human trials began Tuesday at the NIH Clinical Center in Bethesda, Maryland, to be followed by tests at Oxford University in England. If those are successful, volunteers on the ground in West Africa could get the vaccine by October.

The new vaccine acts like a trojan horse – it’s a harmless chimpanzee virus with a few Ebola genes mixed in. Those genes produce markers that tip off immune cells to the threat. The vaccine does not contain the actual Ebola virus.

Robert Garry, a virology expert at Tulane University said scientists are in uncharted territory when it comes to vaccine development.

“There’s no roadmap. You just have to try things and see what work,” Garry said. “You’re not going to expose people to Ebola to see if the vaccine works.”

Ebola is a very powerful virus, Garry said, which is what makes it so hard to treat. Once it enters the body, it’s an arms race between the virus and the immune system. Ebola builds up its arsenal so fast that it usually wins.

The challenge for scientists is to find the sweet spot between between letting Ebola off the hook and sending the immune system off the chain. So far, the animal trials with this vaccine have checked out. Human volunteers will get different doses of the vaccine to compare their reactions.

GlaxoSmithKline is stockpiling 10,000 copies of the vaccine with funding from the United Kingdom, said Donna Altenpohl, vice president of public policy at GlaxoSmithKline and leader of its biosecurity strategy team.

That stockpile might prove worthless if the trials fail, but it’s the only way to ensure that there’s enough vaccine to go around if it works. The World Health Organization asked GSK to accelerate its research after the recent Ebola outbreak.

“It’s a collaboration of everybody jumping in,” including governments, universities and private industry, Altenpohl said. “We’re seeing a global effort to really address a huge public health crisis.”

Stopping the spread of the outbreak is sorely needed in West Africa, where tensions are high both in the countries experiencing the outbreak and those that are on the border. Levine described the situation in Mali as “a powder keg” with surveillance at the border with Guinea to keep Ebola out.

“There’s lots of fear in Mali about Ebola. That’s all people are concerned with at the moment. It’s like ISIS in Iraq,” Levine said.

In impoverished countries like Guinea, Liberia, Nigeria and Sierra Leone, half of those infected with Ebola die. Many of the recent deaths were healthcare workers, who are now reluctant to enter Ebola territory, Levine said.

A vaccine could attract desperately needed help.

Another benefit of the vaccine is that it could be effective with just one dose. That’s crucial in a region with little health care infrastructure.

“There’s excitement, there’s optimism, but also realism that diseases in humans don’t always behave the same way as they do in animals,” Levine said.

The African arm of the study is being conducted at the Center for Vaccine development in Mali, which is a joint venture with University of Maryland and the Ministry of Health.

“Many have called this impossible, but everyone is committed to actually trying to meet this impossible timetable,” Levine said. “So far, every task has been achieved on time or a bit ahead. I can’t believe it all myself.”

Gavin Stern is a national digital producer for the Scripps National Desk. Follow him on twitter @gavinstern or email him at gavin.stern@scripps.com.