Very common since fowleri itself is very common and is generally not geared to actually fight your immune system in any way(since it's an organism where any entry into a body is essentially an evolutionary dead end). In the South that is. Anywhere water doesn't freeze over the winter because it can't survive that.

https://www.sciencedirect.com/science/article/abs/pii/S0014489418300705 - 98 percent seropositive here. Hopefully that resolves some of your fears because as you can see, people interact with it a lot without ill effects. Frankly simple drowning should be a much bigger concern.

What's more scary is how many people in endemic areas in third world countries are seropositive for rabies

https://pmc.ncbi.nlm.nih.gov/articles/PMC11798433/ 11 percent here in Brazilian indigenous communities for those who have not been vaccinated. That's a lot of close calls

Would that mean we could vaccinate for them?