Is an experimental serum saving Ebola-stricken missionaries?

The second American missionary diagnosed with Ebola in West Africa has arrived in Atlanta for treatment. Nancy Writebol landed in Marietta, Ga., shortly before noon en route to a special isolation unit at Atlanta’s Emory University Hospital.

Writebol and Kent Brantly, the doctor she worked with in Liberia, seem to be improving, and many are crediting an experimental drug that has never been tested on humans. It’s impossible at this point to know whether the treatment is the reason for their apparent recovery or whether they are fighting off the virus on their own. While some remain skeptical, details emerging about the serum have health officials cautiously optimistic.

Writebol and Brantly were infected while working in Liberia, one of four West African nations dealing with the world’s largest Ebola outbreak. The World Health Organization said Monday the death toll has increased to almost 900 in Guinea, Sierra Leone, Liberia, and Nigeria. The virus causes fever and bleeding and has a fatality rate of 90 percent without care.

But Brantly walked into the Atlanta hospital Saturday—just three days after Samaritan’s Purse called his condition “grave.” On Monday, CNN released a sensational report crediting the serum with Brantly’s rapid recovery. When the first dose of serum arrived in Liberia, Brantly requested doctors give it to Writebol. Soon after, his condition rapidly deteriorated and his breathing became labored. Brantly even called his wife to say goodbye. But an unnamed source told CNN doctors gave Brantly the serum, and within 20 minutes to an hour, his symptoms had reversed. He felt well enough to take a shower on his own before flying to the United States.

The experimental treatment is called ZMapp, made by San Diego-based Mapp Biopharmaceutical Inc. The serum is a “cocktail of monoclonal antibodies” developed in mice that binds to infected cells and attempts to quarantine the virus, giving the body’s natural defenses a chance. In a statement, the company said it was working with LeafBio of San Diego, Defyrus Inc. of Toronto, the U.S. government, and the Public Health Agency of Canada.

It’s not known when the serum’s development began, but after one successful study with monkeys in 2012, researchers called the serum “the culmination of more than a decade of effort between government and industry partners.” Press releases confirm the involvement of U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID).

Mapp Biopharmaceutical began in 2003, reportedly for infectious disease and biodefense research. Press releases suggest USAMRIID perceives Ebola as a potential weapon: “In addition to being a global health concern, the virus also is considered a potential biological threat agent.”

The drug is made in tobacco plants at Kentucky BioProcessing, a subsidiary of Reynolds American Inc, said spokesman David Howard. The plant “serves like a photocopier,” drastically increasing production. The Kentucky company provided a limited amount of the serum to Samaritan’s Purse—it did not need Food and Drug Administration permission to use the experimental treatment in Liberia. An FDA spokeswoman said she could not say what permission the agency would grant now that Brantly and Writebol are in Atlanta.

Bruce Johnson, president of SIM USA, the Christian aid group Writebol works for in Africa, said after two treatments with the serum, Writebol can now walk with assistance and has regained her appetite. “Her husband, David, told me Sunday her appetite has improved and she requested one of her favorite dishes—Liberian potato soup—and coffee,” Johnson said.

When Writebol arrived in Atlanta, healthcare workers wheeled her into the hospital on a stretcher, suggesting she’s still not well enough to walk on her own. During an afternoon press conference, Johnson said she was weak but that her condition is improving. Before leaving Liberia, Writebol ate some yogurt and was able to stand with assistance when she boarded the plane.

But both U.S. doctors and Johnson said it’s too early to make any miraculous claims about the serum. “Ebola is a tricky virus and one day you can be up and the next day down. One day is not indicative of the outcome,” Johnson said. More testing in humans—Brantly and Writebol are the first—would have to be done before the FDA approved the serum’s commercial production. That could take months or even years.

Meanwhile, concerns continue to skyrocket this week after a Nigerian doctor in Lagos was confirmed to have Ebola. Lagos, Africa’s largest city with 21 million people, equals the populations of Guinea, Sierra Leone, and Liberia combined. The doctor had treated a Liberian-American who died after bringing the virus to Lagos by plane. Three others who participated in the man’s treatment are also exhibiting symptoms.

The Associated Press contributed to this report.