Legionnaire’s Disease, now properly called legionellosis, is the more serious of two disorders caused by the Gram-negative microorganism legionella pneumophila, the other being the milder Pontiac fever. Legionellosis most often occurs in people with incompetent lungs, for example, smokers or older people with chronic lung disorders. Immunocompromised patients are also at risk. Most cases result from breathing water containing the l. pneumophila organism, as from a fountain or cooling tower.
The idea of legionellosis as a major threat somewhat dates the episode; a viewer must remember that when the episode was made in 1977, medicine did not fully understand this illness, and it had barely a year earlier infected over two hundred people and killed over thirty of them. Modern antibiotics have mitigated the hazard of this disease in large measure, but not entirely: as recently as 2005 it killed twenty one of one hundred twenty seven victims in Toronto, Canada.