Legionella are gram-negative, aerobic rod bacteria that can manifest various disease patterns in humans – from flu-like symptoms to severe pneumonia. The germs are naturally found in small numbers in surface waters and groundwater. As an infectious agent, the widespread species Legionella pneumophila is particularly significant.
People with a weakened immune system, seniors or smokers have the greatest risk of infection. According to the CDC, health departments reported about 6,100 cases of Legionnaires’ disease in the United States in 2016.
The bacteria are often found in air-conditioned indoor swimming pools, hotels or hospitals. The seasonal increase in illnesses in the summer and autumn months is due to the increase in leisure and travel activities during this period.
Higher concentrations of legionella tend to grow in hot water tanks of heating systems, air conditioning systems and unused water pipes in buildings at constant temperatures below 55 °C. Legionella find optimal growth conditions in deposits and coatings inside pipe systems. They reproduce best at temperatures between 25 °C and 45 °C (77 and 113 °F). For healthy people, the consumption of infected water is harmless. Bacteria become dangerous when humans inhaled them along with suspended particles/aerosols – for example, during showers, or via humidifiers and air conditioning systems. Then the pathogens can cause severe pneumonia.
Legionella can cause two different diseases in humans: Legionella pneumonia, also called Legionnaires’ disease, and Pontiac fever. People affected with either of these conditions are not infectious.
Legionnaires’ disease is a form of pneumonia that, if properly treated, is easily curable. About two to 10 days after contact with the bacterium, the infected patients experience their first symptoms. The disease initially manifests itself as general discomfort, aching limbs, headaches and chesty cough. If the pneumonia is not treated, or is treated incorrectly, chest pain, chills, up to 40°C (104 °F) fever, diarrhoea and vomiting may occur. Dizziness and confusion occur when the infection reaches the central nervous system. Lung infection with Legionella can last up to four weeks. Although the disease is treatable with antibiotics, 10 % to 15 % of the cases are fatal.
Legionella also causes the so-called Pontiac fever. The infection manifests itself in flu-like symptoms such as fever, discomfort, headaches and aching limbs. Pneumonia does not occur. The period between infection and onset of Pontiac fever is five hours to nearly three days. The disease usually runs its course within a week.