Legionella bacteria is often found in freshwater environments, but becomes a threat when it enters complex, human-made water systems. Such systems tend to allow water to be stagnant for too long and allow the the bacteria to freely spread throughout the system, or have areas that are difficult to clean, allowing the bacteria to re-infect the rest of the system after each attempt to clean it.
There are many different strains of Legionella bacteria, with only a few being dangerous to humans. According to the Journal of Nursing Home Research, the amount of legionella tested in a single water source is not an indicator of risk; amounts can vary greatly even from the same faucet and higher amounts does not correlate with higher risk of infection. Rather, the method to determine the risk of infection is the percent of individual sites (faucets, showerheads) that test positive for Legionella.
The recommended cutpoint based on studies of past outbreaks is 30 percent. Anything higher indicates increasing levels of risk, with below 30 percent indicating low risk. It’s recommended to test at a minimum 10 individual sites, plus 1 site per 100 residents. These tests are recommended to be taken at least once per year.