Naegleria Fowleri meningoencephalitis or the "brain-eating" amoeba

Naegleria Fowleri is a single celled organism living free in hot springs, and in warm bodies of water and in soil, be they ponds, lakes, rivers, or streams.    In rare instances the organism has been known to invade the brains of victims, where the mortality rate from infection is > 95% or slightly less than one survivor in 20 infected.  In water where the organism lives, if a swimmer inadvertently gets water up into the nasal passages, the organism can attach itself to the olfactory nerve.

Hemorrhages and necrosis caused by N. fowleri encephalitis

  It migrates through the nerve endings through the cribiform plate to the olfactory bulb and from there to the rest of the brain.  The organism feeds and proliferates by consuming nerve and brain tissue as it goes along and leaving a necrotic path of destruction. 

The first symptoms is parosmia, which is an inability to identify odors properly, which leads to a total loss of the sense of smell.  From there is progresses to work on the cerebrum, bringing on frank encephalitis symptoms such as severe headache, nausea, and neck rigidity.  When the infection progresses to the brain stem, which controls breathing, death usually occurs.  

Arrows indicate N. fowleri in brain tissue.

By the time the organism is suspected or identified, it is generally too late.  Heroic measures involving high doses of drugs known to affect amoebic infections may be tried even though kidney function may in the process be destroyed. 

Such infections are fortunately quite rare.  The organism is very sensitive to chlorine as well.  There have been less than 300 confirmed cases of this disease as of 2008.   Two persons died in Louisiana while undergoing nasal irrigation in 2011.  A girl died after swimming in hot spring water in Bath, in the UK.in 1978. Nevertheless warnings are posted in areas where the organism is believed to be present.

The Geographic distribution of the disease, as befits a warm water organism is mostly in the southern states, as shown here. And the organism is not limited to just the US, but is present all over the world, though in warmer waters in the summer it is most likely to strike. 

In general, when swimming, don't do anything that might get water way up into the nose, especially in unchlorinated water.

Additional Information:

 http://www.wtsp.com/news/national/article/347184/81/A-deadly-parasite-moving-north

http://www.webmd.com/brain/brain-eating-amoeba

http://www.nbcnews.com/health/deadly-brain-amoeba-infects-first-us-drinking-water-system-8C11172643

http://www.thedailybeast.com/articles/2013/09/23/this-amoeba-eats-your-brain-naegleria-fowleri-rattles-new-orleans.html