In 1976, 4000 members of the American Legion met for their annual convention at a hotel in Philadelphia.

Only three days after the convention, the legionnaires started succumbing to a mysterious illness that resembled pneumonia.
 
The disease quickly became known in the media as the Philly Killer.
An appropriate name for a disease that in a single outbreak would infect 221 and kill 34 people.
 
When the federal officials at the Center for Disease Control published their report on the outbreak a year later, they called the disease by its now commonly accepted name: Legionnaire’s disease.
 
More than thirty years later, Legionnaire’s disease remains a very real threat to people around the world.

The fatality rate of Legionnaire’s disease is between 5% and 30% and between 8,000 and 18,000 people are hospitalized with the disease in the USA every year.  
Globally, the number is obviously much higher.

Legionnaire’s disease is caused by the inhalation of Legionnella pneumophila.
Legionnella pneumophila is a natural bacteria that breathes in temperatures between 77° and 113° F.

It is usually found in biofilm in moist environments, for instance in stagnant water.
Biofilm is a slimy deposit where microorganisms live and breathe and it is highly resistent to disinfectans.
 
When contaminated water is disturbed, for example in a shower, airborne droplets of water called aerosols are formed, which may transfer bacteria to the human respiratory system.