HOUSTON -

The second American with Ebola virus is in Atlanta to be treated as health officials work to figure out just how she and a Texas doctor were infected.

On Tuesday afternoon the U.S. Centers for Disease Control and Prevention says it's still working to complete its investigation. Closer to home, University of Texas Medical Branch-Galveston researchers are continuing to work on a potential medical breakthrough.

An expert says he is encouraged by what he is seeing with the two American Ebola patients, but it is too soon to tell if they'll fully recover. At the same time, UTMB is working on its own vaccine.

Ebola patient Nancy Writebol arrived Tuesday at Emory University hospital in Atlanta. She joins Dr. Kent Brantly. Both were working with the group Samaritans Purse in Liberia when they were infected.

"I'm cautiously optimistic because there's a lot of details that are missing," said Dr. Thomas Geisbert, a professor in UTMB's Department of Microbiology and Immunology.

He says given the information we have the two are doing well with a disease with a 40 percent chance of surviving.

"They're not out of the woods yet... hopefully they will continue to improve. Sometimes patients who have Ebola can appear to be getting better then all of a sudden take a turn for the worse," said Geisbert.

He says he is working with the company Mapp Biopharmaceutical, Inc. that produced the experimental drug. He says UTMB has a grant worth as much as $26 million to help the university to develop vaccines that have proven promising in the lab.

"We've received funds for developing both a preventative vaccines, and also post-exposure treatments and viral drugs," he said.

Geisbert says the only possible positive from this outbreak is that it shows the world that Ebola is a danger and we need more research.