Aspergillus is a common mold genus that humans and animals are continually breathing, but that rarely causes problems. However, in patients with suppressed immune systems, or if introduced directly into the spinal column, the fungus can be deadly. Despite the availability of antifungal agents, aspergillosis in the central nervous system carries a poor prognosis. Though this case sparked the nationwide investigation, as of November 4, the CDC said A. fumigatus had been identified in only one patient. Almost all cases in the outbreak involved a different fungus, Exserohilum rostratum , another common mold that rarely causes problems. 
Despite a 2006 warning letter from the FDA that mentioned the agency was concerned about "potential microbial contamination," former employees said the "clean room" where the vials would have been filled was a controlled space. Workers in the clean room wore blue scrubs, masks and gloves, former employees said. There were usually no more than 6 people in the clean room and one employee said workers handled vials through gloves that were attached to boxed hoods. In order to get into the clean room a worker would have to enter a middle room that was air tight, change clothes and spray their gloves with sterilizer, according to a former employee.