It is natural to find legionella bacteria in freshwater lakes and ponds. Some inevitably gets into domestic supplies and in very small quantities it is harmless. It is when it manages to breed and multiply inside our plumbing systems that it becomes dangerous. The lungs are particularly susceptible. Most people think of a pneumonia-like illness when you say legionella, but it can also cause a variety of problems including “Pontiac fever”, “Lochgoilhead” fever, septic shock and organ failure. Legionella is a killer.