The early symptoms of Naegleria fowleri infection are similar to those caused by other more common illnesses, such as bacterial meningitis . People should seek medical care immediately whenever they develop a sudden onset of fever, headache, stiff neck, and vomiting, particularly if they have been in warm freshwater recently.
Naegleria fowleri is a heat-loving (thermophilic) organism. It grows best at higher temperatures up to 115°F (46°C)
http://www.cdc.gov/parasites/naegleria/general.html
I doubt many people are swimming in 115 deg. water.
So, 100? 90?
It turns into a cyst at 50 deg. Can the cyst warm up in your body? Can it survive freezing?
Granted, it’s about as rare as rabies, and possibly more survivable. But at least you generally know with rabies that you’ve been bit.