In the grand scheme of things that can kill you, Naegleria fowleri sounds pretty frightening. When it finds itself in a swimmer’s nose, this freshwater amoeba wiggles its way up the olfactory nerve to the brain. There, it starts destroying brain tissue. But, as Laura Sanders reports for Science News, the brain eating might not actually be the thing that kills you when you get an N. fowleri infection.

Stomach acid is deadly to the amoeba, so the nose is the its only a shot at a successful colonization of its host. Upon entering the brain N. fowleri sets off alarm signals in the body’s immune system, explains Sanders. This triggers inflammation, which is what causes the brain to swell, and this may pave the way for the pathogen’s destruction. The first signs of infection seem fairly innocuous — headache, nausea and fever — but more severe symptoms follow, including hallucinations, seizures and brain swelling.

And it's that immune reaction and brain swelling that might actually be the real killer here. In fact, Abdul Mannan Baig, a physiologist at Aga Khan University in Pakistan, reported in the journal Acta Tropica that the amoeba takes hours longer to destroy brain cells in the absence of immune cells, writes Sanders.

Here’s what Baig thinks is going on: The swelling disrupts the blood brain barrier — the system that lets things in and out of the brain — and actually causes brain damage. At the same time, the amoeba releases enzymes and toxins that make that brain damage worse and ultimately irreversible.

Cases of N. fowleri are rare but predominantly fatal. In 2013, a 12-year-old girl became the first survivor in decades. Doctors approached her case with a focus on reducing brain swelling, and if Baig is right, that could explain why it worked.