Legionnaires' disease is primarily transmitted by inhaling small droplets (aerosols) of water that contain Legionella bacteria. These droplets can come from sources such as air conditioning cooling towers, hot and cold water systems, showers, fountains, humidifiers, whirlpool spas, and other water systems where the bacteria can grow. People can also become infected by accidentally aspirating (inhaling) contaminated water into the lungs while drinking. The bacteria thrive in warm water environments, typically between 20°C and 50°C (68°F to 122°F), with an optimum temperature around 35°C (95°F). Transmission from person to person is generally not known to occur, although there has been an extremely rare suspected case where a close caregiver may have transmitted it. Most people exposed to Legionella bacteria do not get sick; those at higher risk include older adults, smokers, and people with weakened immune systems or certain chronic health conditions. In summary, Legionnaires' disease is transmitted mainly through inhalation of contaminated water aerosols or aspiration of contaminated water, not through direct human contact or from other people.