(CNN) -

Two American missionaries infected with the deadly Ebola virus were given an experimental drug.

Shortly after, Dr. Kent Brantly and Nancy Writebol were flown separately from Liberia to Atlanta's Emory University Hospital -- the first human patients with Ebola to ever come to the United States.

Writebol was released from the hospital Tuesday, a source close to the Writebol family told CNN. On Thursday, Brantly walked out of that same hospital with no signs of the virus of his system, Emory spokesman Vince Dollard said.

Their recoveries seem to offer hope for those fighting the largest Ebola outbreak in known history. More than 2,400 people have been infected by the virus, according to the World Health Organization, and it's killed more than half.

But ZMapp is not an approved treatment for Ebola; in fact, no approved, proven treatment exists. So governments, aid organizations and scientists around the globe are racing to find a way to stop the virus.

Here are answers to questions about Ebola patients and treatments for the disease.

1. Are Brantly and Writebol cured?

Mostly. For Ebola patients to leave isolation, two blood tests had to come back negative for the Ebola virus. So their bodily fluids, like blood, sweat and feces, are no longer infectious.

However, some doctors believe the virus can remain in vaginal fluid and semen for up to several months, WHO says.

2. Are they now immune to Ebola?

Doctors believe surviving Ebola leaves you immune to future infection. Scientists have found that people who survive Ebola have antibodies in their blood that would provide protection against that strain of the virus in the future, and possibly against other strains as well.

But, as you can imagine, they haven't tested this theory by infecting survivors with the virus again.

There are four Ebola strains known to infect humans; the Zaire ebolavirus causing the current outbreak is the most common.

3. Who else has been given ZMapp?

The Ebola drug was flown to Spain to give to a priest named Miguel Pajares, who had contracted the virus in Liberia. Pajares died on August 12. It's unclear if he was given the drug before he died.

ZMapp appears to be helping three Liberian health care workers who were given the experimental drug. They have shown "very positive signs of recovery," the Liberian Ministry of Health said earlier this week. Medical professionals treating the workers have called their progress "remarkable."

4. Who makes the drug?

The drug was developed by the biotech firm Mapp Biopharmaceutical Inc., which is based in San Diego. The company was founded in 2003 "to develop novel pharmaceuticals for the prevention and treatment of infectious diseases, focusing on unmet needs in global health and biodefense," according to its website.

Mapp Biopharmaceutical has been working with the National Institutes of Health and the Defense Threat Reduction Agency, an arm of the military responsible for countering weapons of mass destruction, to develop an Ebola treatment for several years.

5. How does ZMapp work?

Antibodies are proteins used by the immune system to mark and destroy foreign, or harmful, cells. A monoclonal antibody is similar, except it's engineered in a lab so it will attach to specific parts of a dangerous cell, according to the Mayo Clinic, mimicking your immune system's natural response. Monoclonal antibodies are used to treat many different types of conditions.

Sources told CNN the medicine given to Brantly and Writebol abroad was a three-mouse monoclonal antibody, meaning that mice were exposed to fragments of the Ebola virus, and then the antibodies generated within the mice's blood were harvested to create the medicine.

However, the drug can also be produced with proteins made from tobacco plants. ZMapp manufacturer Kentucky BioProcessing in Owensboro provided limited quantities of this kind of the drug to Emory, according to company spokesman David Howard.

6. Did doctors know it would work in humans?

No. The drug had shown promise in primates, but even in those experiments, just eight monkeys received the treatment. In any case, the human immune system can react differently than primates', which is why drugs are required to undergo human clinical trials before being approved by government agencies for widespread use.