Still, the virus has caused "at least several hundred cases" of human disease in regions where it is common, said NIH's director of the Office of Biodefense Research Affairs, Michael Kurilla.
"The mortality is anywhere from at least 10 to 20 percent or slightly more," Kurilla told ABCNews.com, adding that there is no treatment or cure for Guanarito. "That is considered very, very severe if you have a 1 in 5 chance of dying without anything to do for the person other than provide supportive hospital care."
Kurilla said the Galveston biolab requires the most stringent safety measures because it studies biosafetly level BSL-4 materials, or dangerous infectious diseases that have no vaccines or cures. BSL-4 materials include Guanarit, Ebola and smallpox.
The Galveston researchers were conducting a routine inspection on March 20 and 21 when they noticed there were only four Guanarito vials instead of five. They announced the lapse on March 23.
The university does not believe this was the result of a security breach or any wrongdoing, but it notified the Centers for Disease Control and Prevention.
All solid waste in BSL-4 labs is typically disposed of via a pressurized heating process that destroys hazardous materials without allowing the liquid to boil away, Kurilla said. As such, it's unlikely that investigators will be able to determine and prove whether this is what actually happened to the vial.
It's possible investigators will find the clerical error that led to the accidental disposal, but with computerized record keeping, it's less likely that vial numbers were transposed and the error can be easily traced, Kurilla said.
"I suspect that they may not ever be able to account for it if it was that kind of human error," Schaffner said. "This is a record-keeping issue, which means it was a human issue, which means doing that kind of tedious, important work, there was just a momentary slip up."