The term “outbreak” has significant meaning in terms of Legionnaires’ disease cases, so it is important to fully understand the implications of an outbreak of Legionnaires’ disease versus a “sporadic” case.
Sources of exposure to Legionella bacteria for Legionnaires’ disease patients oftentimes include public areas, such as hospital water systems, hotel water systems, and other large scale water supplies. Therefore, many times multiple people contract the disease in similar circumstances when a building has a water supply contaminated with high levels of Legionella. Generally, state and local health departments become involved when two or more cases are reported that have a common source and a common timeframe. When this cluster of cases exists for a given location, the situation can be considered an outbreak.
On the other hand, Legionella is an ubiquitous organism, so exposure to the bacteria and contraction of Legionnaires’ disease is certainly possible on a remote basis, when only one person gets sick at a given location in a given time period. These sporadic cases are generally not investigated by municipal health departments in the way that outbreaks are.
The question of whether a patient’s case is sporadic or part of an outbreak has important affects on Legionnaires’ disease litigation. When a patient’s case is part of an outbreak, epidemiologists can point to a cluster of people who became sick at same location in a similar period of time and use probability to estimate whether the patient contracted the disease at that source. If the patient did not have other potential exposures at other high-risk locations, this can be a crucial argument that the patient contracted it as a part of the outbreak. Also, when health departments become involved, as they generally do in such outbreaks, they conduct investigations of the likely source, including Legionella sampling, which can help explain locations and concentrations of Legionella within a water system. This information is very useful during trial, and is generally not available for sporadic cases.