Samples of Ebola are in short supply for U.S. scientists who require a fresh, steady stock of the virus to track its changes and to plan ahead for new drugs and vaccines. Similar to the flu virus, Ebola mutates, however slightly, as it spreads, and keeping up with those changes is key to stopping new infections and heading off future outbreaks, according to Reuters.

"No one really knows right now what has the virus mutated to or if it has mutated," Charles Chiu, a microbiologist and infectious disease expert at the University of California, San Francisco, told Reuters. “We're not going to be able to determine in advance whether or not it has changed to a form where it might evade diagnostic assays or might render current vaccines or drugs ineffective” without new samples of the virus, he said.

Moving Ebola samples safely from one place to another is complicated, and transport companies have been wary of working with Ebola because of growing concerns about a U.S. spread of the virus. Doctors in Guinea, Liberia and Sierra Leone, where infections have topped 13,500 people, have been slow to hand over samples of Ebola, scientists told Reuters, which means U.S. disease specialists studying the virus at eight major research institutions are not getting the latest specimens.

As Ebola continues to spread across West Africa, it has more chances to change and adapt. Scientists identified 341 mutations of the virus as of late August, according to a study published that month in Science. One fear is that the virus could become more transmissible by becoming less pathogenic. A virus that sickens and kills its victims more slowly would leave more time for patients to infect others, National Geographic reported. The biggest nightmare, scientists have said, would be if the virus went airborne, something experts believe is highly unlikely but not impossible.

Since the Ebola outbreak in West Africa began in March, health officials have struggled to keep pace with the ever-changing situation on the ground. The Ebola virus is "moving at the speed of sound," James Dorbor Jallah, the national coordinator of Liberia's Ebola Task Force, said in August, but international aid is moving at the "speed of a snail." Health officials in Sierra Leone announced Tuesday that the virus had broken out in a new area of the country, raising concerns about a new chain of infections.