In 1976, 29 people attending a Legionnaire's convention in Philadelphia died from a mysterious illness. After months of investigation, the root cause of their deaths was traced to a bacterium called legionella, or more precisely Legionella pneumophila. The legionella bacteria thrives in standing water sources such as ponds, wells, cooling towers, air conditioner ducts and hot tubs. Although the original source of the legionella infestation was thought to be stagnant water collecting under air conditioning units, further testing suggested that this was not the main contributor.

Legionella derives its scientific name from the association with the Legionnaire's outbreak. Many of us have heard of Legionnaire's disease, but Legionella pneumophila is also responsible for a similar respiratory infection called Pontiac fever. Both illnesses are distinguished by their severe flu-like symptoms and gastrointestinal distress.

Legionella does not infect victims through the act of drinking contaminated water or through skin contact alone. Legionella is spread as an aerosol, meaning the bacteria is contained in small water droplets. If the legionella-infected mist can get past the throat's natural choking mechanism, it can be aspirated into the lungs.

Once legionella enters the lungs, it does what other bacteria do -- it begins to feed on living tissue. The body sends out white blood cells to counteract the invading bacteria, but the result is often a growing infection and excessive mucus. Legionella pneumophila, which is responsible for 90% of all legionella infections, does not always respond well to penicillin-based antibiotics, which means the patient is often given a higher strength antibiotic cocktail to kill off the legionella. If the patient is a heavy smoker or has a compromised respiratory system, the infection could become fatal. Many of the original 29 victims in 1976 were heavy smokers.

Legionella bacteria are known disease agents, so municipal water supplies are routinely tested for their presence. Many people exposed to legionella never develop any symptoms of Legionnaire's disease. Water contaminated with legionella must be breathed into the lungs, not swallowed, in order to become problematic. Heavy smokers are more at risk because their choking mechanism doesn't work as well, leaving their lungs more vulnerable to legionella-infested droplets of water. Some experts suggest limiting exposure to hot tubs and misting towers in order to avoid exposure to legionella and other aerosol-delivered infective agents.