A free living fresh water amoeba. N. fowleri. (PLoS Review)
Swimming in fresh water. It runs up the 1st nerve through the cribriform plate and thence into the brain.
It can be found in tap water, including the US (PubMed)
Thanks to global warming, it is now as far north as Minnesota, oh yeah sure doncha know (PubMed).
High-dose intravenous and intrathecal amphotericin B. Not that it will work.
Most die, those who live usually have extensive brain damage. It isn't pretty.