The disease, which has claimed the lives of two people in Merthyr Tydfil area of Wales this week, kills up to 40% of people who contract it.

The bacteria that cause disease, called Legionella, commonly live in lakes and streams in low numbers, but thrive in the slimy "biofilm" that forms inside pipes and water tanks.

The bacteria spread to people when they inhale droplets of contaminated water from cooling towers, sprays, and drinking water.

Municipal water systems commonly use free chlorine to disinfect drinking water.

Free chlorine kills bacteria quickly, but its effect is fairly short-lived.

It often does not retain its bacteria-killing ability all the way to the tap and does not penetrate into the biofilm very well.

The slower-acting monochloramine better maintains its disinfecting power as the water travels through the water system, and can penetrate better into the biofilm where legionella and other microbes can hide.

Dr Jacob Kool and colleagues of the US Centers for Disease Control and Prevention compared the disinfection methods used by water systems that supplied 32 hospitals which had documented outbreaks of Legionnaires' disease with those supplying 48 hospitals which had not had outbreaks.

Writing in The Lancet medical journal, they say: "We found that hospitals supplied with water containing only free chlorine were 10.2 times more likely to experience an outbreak associated with drinking water."

Assuming that about half of Legionnaires' cases are due to contaminated drinking water, the reseachers calculated that routine treatment of municipal water systems with monochloramine could save between 900 and 2025 lives each year in the USA alone.