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It’s easy to assume nothing is wrong inside your pipes if your water looks clear and tastes normal. Most people overlook the effects of Legionella in water for exactly that reason. It’s a kind of bacteria that can build up inside home plumbing and water heaters without ever making any tangible change to how your water looks or tastes, so it comes as no surprise that little is known about it. Getting sick from it takes a more subtle kind of exposure than simply drinking it, and given the number of rising cases across the country, it’s something that should be on your radar.
That single fact drives everything else in this guide.
Below, you will learn where Legionella tends to grow inside a home, why plumbing and water heaters create the right conditions for it, and what you can do to keep your risk low.
Legionella is a type of bacteria that occurs naturally in freshwater sources such as lakes and streams. It travels into your home through the public water supply, where it typically stays at low levels that cause no problems at all.
But the trouble starts when the bacteria find warm, still water inside pipes, tanks, or fixtures. Those conditions let Legionella multiply far beyond its normal levels.
Once that happens, you don’t get sick by swallowing the water, but by breathing in the fine mist or aerosol that carries the bacteria into your lungs.
Legionella does not spread evenly through your home’s water supply, but here are the areas most likely to support bacterial growth:
Your water heater can develop temperature stratification, where the water at the bottom of the tank stays cooler than the thermostat setting suggests. Sediment tends to collect at the base too, creating warm, low-flow pockets where bacteria can settle in and grow undisturbed.
Several spots in a home often sit unused for weeks at a time, such as:
When water stays still for that long, it quickly loses its disinfectant residual, removing one of the main barriers that would otherwise keep bacteria in check.
Faucet aerators, showerheads, hot tubs, and humidifiers all break water into fine droplets. A fixture producing mist or spray puts more Legionella into the air you breathe than a fixture that releases a steady stream.
Biofilm is a thin, slimy layer of bacteria and organic material that builds up on the inside of pipes over time. Legionella can shelter inside it, gaining protection from the disinfectants used in municipal water treatment. That shelter is also why treatment at the source rarely reaches bacteria embedded this deep in your household plumbing.
Each of these spots creates the same basic problem: warm, still water with nowhere for disinfectant to reach.
Warm water and still pipes work together with worn plumbing to give Legionella exactly what it needs to multiply. Here is why each factor plays a part:
Legionella thrives in water between roughly 77°F and 113°F (25–45°C). Many water heaters land within or close to that range, especially when the tank is set lower to reduce scald risk or save on energy costs.
When water sits still for an extended period, it loses its chlorine or chloramine residual. Reduced circulation also lets the water’s temperature drift, which creates small warm pockets that are ideal for bacterial growth.
Scale buildup gives bacteria more surface area to attach to and hide within. Once biofilm forms over that scale, water treatment chemicals have an even harder time reaching the bacteria living underneath.
Older plumbing tends to carry more corrosion and sediment than newer systems, along with more irregular flow. Those conditions reduce circulation and give bacteria more places to settle in undisturbed.
Lowering your risk comes down to a handful of consistent habits around temperature and water flow, plus routine cleaning, and fortunately, none of them require special tools or a large time commitment. Here are the most practical steps you can take:
Store hot water at 140°F (60°C) or higher to keep Legionella from establishing itself inside the tank. Deliver water above 122°F (50°C) at the tap for the same reason. If scald risk worries you, a thermostatic mixing valve installed near the point of use lets you keep storage temperatures high while protecting your family at the faucet.
Run showers and faucets weekly, even in guest bathrooms or seasonal properties. Keeping water moving prevents the stagnation that lets bacteria build up in the first place.
Remove and clean showerheads and aerators every few months to clear away mineral scale and disrupt biofilm before it thickens. A vinegar soak or an appropriate disinfecting solution works well for this kind of routine upkeep.
Store cold water below 68°F (20°C) whenever possible, since cooler temperatures discourage the warm conditions Legionella needs. Insulating cold water lines that run near heat sources can help you maintain that temperature.
Legionella exposure can lead to one of two illnesses, and which one you get shapes what symptoms to expect:
Legionnaires’ disease is a severe form of pneumonia. Watch for fever, chills, a persistent cough, and muscle aches. Some high-risk groups for this include:
Pontiac fever feels more like the flu and does not involve pneumonia. It tends to pass faster, though, so most people recover without specific treatment.
Either illness usually shows up two to fourteen days after exposure. If you notice these symptoms after possible contact, seek medical evaluation right away. But symptoms alone cannot tell you where the exposure came from, so if you suspect your water is the source, make sure to do a water test.
You cannot detect Legionella with your senses, so a quick look or taste test at the tap will not tell you anything useful.
The bacteria produce no visible cloudiness or odor, and they leave no noticeable taste, even at concentrations high enough to make you sick.
A few circumstances raise the odds of Legionella growth, such as a long plumbing shutdown, a home reopened after sitting vacant, an older plumbing system, an immunocompromised household member, or a past history of contamination in the building. None of these guarantee a problem on their own. But suspicion is not the same as confirmation, and testing remains the only reliable way to know for certain.
A confirmed result typically calls for a few clear steps:
Avoid aerosol-generating fixtures such as showerheads and hot tubs while remediation is underway. This cuts down your chances of inhaling contaminated droplets during the process.
Thermal flushing raises water temperature throughout the system to kill bacteria. Hyperchlorination uses elevated chlorine levels to reach a similar result. Both methods typically call for professional handling to apply safely and effectively.
Follow-up testing confirms whether the treatment worked. From there, ongoing maintenance keeps the same conditions from developing again.
Yes, and that is exactly why showers deserve extra caution. Showers produce fine aerosols that are easy to inhale, which is a very different exposure from drinking the same water. If you or someone in your household falls into a higher-risk group, such as older adults or people with weakened immune systems, treat a confirmed positive result as a reason to avoid showering until the system has been addressed.
Rarely. Legionnaires’ disease develops from inhaling contaminated droplets. Swallowing contaminated water carries a much lower risk by comparison. One exception is aspiration, when liquid enters the lungs during swallowing, which can pose a concern for people who have trouble swallowing safely. Outside of that specific case, aerosol exposure from showers and faucets remains the leading concern with Legionella in water.
A few consistent habits go a long way here: maintain proper water heater temperatures, prevent stagnation, flush unused fixtures on a regular basis, and clean showerheads and faucet aerators every few months.
If you have specific concerns or higher-risk occupants in your home, go with professional testing to save yourself time and money.
Most homes will never run into a Legionella problem, but now that you know how the bacteria behaves, you have what you need to make informed decisions and keep your plumbing system healthier for years to come.
If you have questions about Legionella in your water system, ETR Laboratories provides ISO/IEC 17025-accredited water testing and laboratory analysis to help you get clear answers. Contact ETR Laboratories today to schedule a water test.
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