Naegleria fowleri is a free-living ameba: a single-celled organism that lives in soil and warm fresh water around the world. It is often called the "brain-eating ameba" because, in rare cases, it can infect and destroy brain tissue. The infection is very rare — but it is almost always fatal, and that is exactly why awareness matters.
What it is
- It is a naturally occurring ameba that thrives in heat — it is part of the environment, not a sign of pollution.
- It causes a brain infection called primary amebic meningoencephalitis (PAM).
- Infection is rare: typically fewer than 10 people a year get PAM in the United States.
- Between 1962 and 2024 there were 167 reported U.S. cases — only 4 people survived.
Where it lives
It thrives in heat and is a natural part of the environment — not a sign of dirty water. It can be found in:
- Warm fresh water — lakes, rivers, ponds, and hot springs
- Soil, including the sediment at the bottom of lakes, rivers, and ponds
- Naturally hot (geothermal) water, such as hot springs
- Swimming pools, splash pads, and surf parks that are poorly maintained or under-chlorinated
- Warm water discharged from industrial or power plants, and even water heaters
- Tap water, in some cases
How infection happens
Infection happens in only one way: when water containing the ameba goes up the nose and travels to the brain. It usually happens while swimming or diving in warm fresh water — most often after long stretches of hot weather, when water is warmer and levels are lower. A few infections have come from tap water used to rinse the sinuses or nasal passages.
What you cannot get it from
This part matters just as much. You cannot get this infection from:
- Swallowing or drinking water that contains the ameba
- A properly cleaned, maintained, and chlorinated pool
- Another person — it is not contagious and does not cause outbreaks
- Shower mist, humidifier vapor, or other water vapor (there is no evidence of spread this way)
Who is most at risk
Most people who have gotten PAM in the United States have been young boys — possibly because of activities like diving and playing in shallow sediment. Historically most cases were in southern states, but the map is shifting north: since 2010, cases have been confirmed as far north as Nebraska, Iowa, Minnesota, Indiana, Maryland, and northern California. Warming waters may be part of why.
Source: Medical and prevention information on this site is drawn from the U.S. Centers for Disease Control and Prevention (CDC). Always follow current CDC guidance and consult a medical professional. Information on this page reflects the CDC Naegleria fowleri pages reviewed on 2026-08-04.