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Legionnaires' Disease, in Plain English

It is a bacterium, not a mold — and it is the other invisible thing that grows in gym water. What Legionnaires' disease is, where Legionella hides in a gym, why Germany tests for it by law every year, and why nobody can say what bacteria were in the Daytona water: it was never tested.

If you have read our Gym Watch pages, you have seen one diagnosis come up again and again: Legionnaires’ disease. Kennewick. Naperville. Boise. Ocoee. Thomas asked the obvious question — what actually is that? — and it deserves a plain answer, because it explains half the pattern on this site.

Education, not medical advice. This page explains what health agencies have published about Legionella and Legionnaires’ disease. It does not diagnose anyone. If you develop pneumonia-like symptoms after using a gym’s showers, hot tub, or steam room, see a doctor promptly — and tell them where you were.

First: it is not mold

Mold and Legionella get lumped together because both grow in badly kept wet buildings. They are different organisms, living in different places, found by different tests:

A gym can have both problems at once, in different parts of the same building: mold on the surfaces, Legionella in the water. Testing for one tells you nothing about the other. A clean mold swab does not clear the water, and a clean water test does not clear the benches.

What the disease is

According to the CDC, Legionnaires’ disease is a serious type of pneumonia — a lung infection — caused by Legionella bacteria. The same bacteria also cause a much milder illness called Pontiac fever: flu-like fever, muscle aches, and headache, without pneumonia, which usually passes on its own.

The CDC’s plain facts:

Where it hides in a gym

Legionella exists naturally in freshwater, in numbers too low to matter. It becomes dangerous inside a building’s water system, where conditions let it multiply — and a gym is a stack of those conditions:

The delivery route is the whole trick: nobody has to touch anything or drink anything. You just have to breathe in the room.

When it gets found, this is what it looks like

Legionella leaves a paper trail only when people end up in a hospital and a health department is forced to investigate. When that happens, the arc is consistent — cases, a positive water test, a closure:

Notice what every one of those records has in common: a hospital did the detecting. There is no routine tripwire in front of the hospital — no scheduled test that catches the bacteria while it is still just bacteria. Which brings us to Daytona.

The question nobody can answer about Daytona

People ask what bacteria were in the water at the Gold’s Gym in Daytona Beach. Here is the honest answer: nobody knows, because nobody ever tested it. The only testing ever done at that facility was Thomas’s own consumer mold kit, swabbed in the sauna in August 2024 — and it came back with three mold species: Chaetomium, Aspergillus, and Nigrospora. That kit tested for mold, on surfaces. No bacterial testing of the facility’s water was ever performed — not by the gym, not by any agency — because no mandate requires it. The question “what bacteria were in that gym?” has no answer. Not “the tests came back negative.” There were no tests. That empty space where an answer should be is not a gap in the story. It is the story.

Other countries test for this. Florida doesn’t.

In Germany, this exact question has a legally required answer. Under the Trinkwasserverordnung (the federal Drinking Water Ordinance), operators of commercial shower systems — gyms included — must have their water tested for Legionella by an approved laboratory at least once a year. If a sample reaches the legal action value, the operator must notify the local health office immediately, investigate, produce a written risk assessment, and carry out protective measures — up to and including shutting the showers down. Switzerland’s rulebook names fitness studios explicitly. We lay the whole map out in The World Regulates This.

Florida regulates pool chemistry. It does not require Legionella testing of a gym’s water — not annually, not ever. So at Daytona, the bacteria question was never asked, and “we don’t know” became the permanent, official answer.

What you can do with this

You cannot test a gym’s water with a home kit — Legionella culture testing is laboratory work. What you can do is what this site exists for:

Sources