The Legionella bacteria was first discovered in 1976 as the end result of the Legionnaires Disease outbreak in Philadelphia which caused 200 cases, and was found to be the cause of a similar outbreak the previous year at the same hotel as well as a series of mysterious epidemics going back 50 years. Although legionella was found in the cooling towers on the roof and in a local pond, the eponymous outbreak was never actually traced to a ventilation system design flaw and may have been due to peculiarities of outdoor airflows. Legionella has the unusual characteristic of causing two diseases, Legionnaires Disease and Pontiac Fever. Factors influencing susceptibility include the elderly and those with suppressed immune systems, heavy smokers and alcoholics, and others with weak lungs or constitutions.
Males are over twice as susceptible as females.
It incubates in human hosts within 2-10 days and will not abate without medication. There have been over 50 separate outbreaks. This variant made its first appearance in Pontiac Michigan in 1968 in the Oakland County Health Department where 95 of 100 employees and 49 of 170 visitors became sick. Three CDC investigators were sent into the building and they became sick. They were replaced with three more investigators who also became sick.