I heard that we can't even die here. [Reid furrows his brow, pursing his lips.] Is that... How is that even possible? Do you have any idea?

I think Thing is a Paras. I read a little about them in Cherrygrove. Apparently, tochukaso is endemic to the species, to the point where people actually consider the state of being utterly taken over by the fungus to be a Paras's evolution. But, there have apparently been cases of Paras removing it on their own and being able to keep it from growing back, so there has to be a way.