The Fear Virus was a virus developed by the Separatists during the Clone Wars. It was a relatively simple virus and so could be killed by most antiviral compounds, such as those found in a starship's bio-scrubbers, as well as by exposure to vacuum. It had both a solid, slime-like form as well as an airborne form. When beings were exposed to the airborne form, it took one hour for the virus to affect them, though it was possible for beings to resist it at this time, and it would affect them more and more each hour. The slime-like substance, however, affected anyone who came in contact with it.
The virus itself attacked the victim's brain, causing hallucinations which were generated from the more morbid regions of the victim's mind. These illusions were very dangerous, however, as the victims were essentially hypnotized and so would fully believe that any fantastic creature from their imagination, or past, that they see was real. Any damage that the illusionary creature dealt them would thus cause actual damage, generally in the form of blisters, cuts, and bruises. However, the cure for the virus was in the victim's subconscious; squirt guns, whipped cream pies, odd gestures, and other nonsense that children would use to ward off or dispel their imaginary monsters would also work against these illusions.
The only known containers of the virus were placed upon a bulk freighter which drifted in space for over one hundred years before being found. Somehow during this time it had become a burned-out wreck with holes in its hull through the bridge and engineering sections. Also, for over 75 years before being found, the ship's bio-scrubbers had been overloaded. Though the group entered the ship and were attacked by both the virus itself and the droids on board, they managed to make it off alive and decontaminated the entire ship of the virus, taking it as their own.