courtesy of Petr Dlouhý
Along the entire roughly 3 to 9 foot tall plant are stinging hairs that can embed themselves in your skin. These stingers emit an extremely painful neurotoxin. The sting causes a cluster of red dots that eventually spread until they grow together. The intense pain is known to be strong enough to put people and animals out of their minds. It can go on for months. What is worse, allergic reactions to the plant may get worse with repeated exposure, as with researcher Marine Hurley.
Stings from the gympie gympie can allegedly kill humans and large mammals. That means victims should get the stinging hairs out as soon as possible and seek palliative treatment. You can remove the stingers with a hair removal strip. Most people don't have those lying around while hiking, but being aware of your surroundings and being prepared goes a long way, so start if you know these plants are nearby.