Legionella Disease can be fatal and is a form of pneumonia. There also less serious diseases arising from the same source Pontiac fever and Lochgoilhead fever. Older people and those with existing health issues or reduced immunity are at greater risk. The disease is caught when the bacteria are inhaled via water droplets in the air. The bacterium Legionella pneumophila is a common one found in natural watercourses and ponds and it is rarely a cause of illness in its natural habitat. Your dawn swim in the local river is not likely to result in this particular illness. A shower is a much more dangerous activity because the water sprays into droplets that you might breathe in.