Naegleria Fowleri (en.wikipedia.org)

One of the worst outbreaks ever documented took place in Czechia, in a swimming pool where, unknown to the management, a cavern full of stagnant water formed behind one end of the pool. 16 young people died in a span of a few weeks.

There was recently a case of this in Louisiana. The 4th since 1937. It's exceedingly rare.

https://www.cidrap.umn.edu/misc-emerging-topics/louisiana-co...