How does legionella spread?

Legionella spreads in water, and it causes harm when contaminated water is broken into a fine spray that can be breathed in. It does not pass from person to person, and it does not travel through the air on its own: the bacterium needs a droplet of water as its vehicle, and the droplet needs to be small enough to inhale.

That single fact explains the shape of every control in the guidance. The hazard is not the water in the pipe; it is the water in the pipe after it has been allowed to multiply, and then only at the moment it is turned into an aerosol. Control therefore works backwards: stop the multiplication in the system, and manage the outlets that make the spray.

The spray-producing outlets

Any fitting that breaks water into droplets is a potential release point. Showers are the classic example in homes and workplaces, because they produce a fine mist at breathing height. Spray taps, hose pipes, pressure washers, ornamental fountains, spa pools and cooling towers all do the same job, some of them at much larger scale. This is why shower heads and hoses get a cleaning and descaling regime of their own, why cooling towers carry notification duties, and why spa pools have dedicated guidance. The outlet is where a water-hygiene problem becomes an exposure.

How it spreads inside the building

Within a building, legionella spreads from reservoirs. Trace numbers arrive with the incoming mains supply and settle wherever the conditions suit them: a warm branch that is rarely drawn, a cold water tank that has warmed up, scale on a pipe wall, the biofilm layer that lines the whole system. Established in those spots, the bacteria multiply, and ordinary water movement carries them through the pipework to every outlet downstream. A single colonised dead leg or little-used outlet can seed a whole run of pipework, which is why finding and removing or flushing those branches matters as much as any other control.

It also explains a point that surprises people: flushing is itself a release of spray. Running a colonised outlet pushes contaminated water through a spray-producing fitting, so the control measure has to be carried out gently. Open the outlet slowly, remove the shower head or hose first where you can, and ventilate the room. The guide to flushing little-used outlets covers the technique.

What does not spread it

It is worth being explicit about the negatives, because they shape what you do and do not need to control. Legionella is not contagious between people. It does not spread through intact skin contact with water. And the control regime is built around inhaled spray, not around drinking. All of the effort goes into the water system and its aerosol-producing outlets, not into separating people from each other.

For a duty holder, the controls map directly onto the two steps. Stop the multiplication with temperature, turnover and cleaning: hot at 50°C or more at outlets, cold below 20°C, weekly flushing of little-used outlets, regular descaling of spray fittings. Then record each control in a log book, because a control you cannot evidence is, on paper, no control at all. The legionella risk assessment tool structures the walkthrough that identifies your spray outlets and stagnation points, and what causes legionella explains the growth conditions the controls remove.