It was recently discovered that poison ivy is actually a psychologically transferred allergen. That is, you don’t get a rash from touching it, but from merely seeing it and thinking about it. The rash localizes in the location where you think you touched the plant. In severe cases, people can actually think themselves into contracting a poison ivy rash even though the closest plant is miles away.
In the late 1980s, there were attempts to weaponize the psychological behavior of poison ivy. Ultimately, though, it couldn’t be controlled as everyone involved with the project suffered from a severe case of “brain itch”. Treatment was simple—a heavy application of calamine lotion and vodka—but it left subjects with a jumpy kind of “itch paranoia” for the rest of their lives.