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.